Coach Daniel Matheny has some great tips for building fitness by injecting running workouts into your routine.
Get on Your Feet
While you don't want to give up cycling-specific training completely during the offseason, adding other training modes can keep you mentally fresher and physically stronger so that you'll end up performing better.
Days like this are great for getting out for a run.
Running is one of my favorite offseason activities for most cyclists as it is the simplest and most basic form of endurance training. It can also be done most anywhere and with a bare minimum of equipment and running isn't limited by most weather conditions.
As a cyclist, or anyone else that doesn't experience much weight-bearing activity in their sport (ie swimmers), or if you've simply detrained through the fall, I recommend run/walk workouts on trails where you can run the flatter sections and walk the hills. This will help you reduce the eccentric-related soreness from running downhill while also keeping your heart rate down so it's truly a low-intensity session. Alternatively, if the conditions permit, try snowshoeing. It's low impact and can be a good aerobic base activity if you keep moving.
If you need a high-intensity workout, hill repeats are great as they build strength while minimizing impact. A word of warning, however. Most cyclists have cardiovascular engines that can outperform their chassis (musculoskeletal system) so it's a must to start easy when you begin running. If you find yourself crippled from soreness a day or two after a workout, you've overdone it. Be forgiving with your expectations and allow your body time to adapt. It's usually better to do shorter, more frequent workouts than to go overboard and not be able to train again for a handful of days due to inflammation.
You don't need to run Pikes Peak to get the benefits of hill repeats (but it doesn't hurt!)
Begin with 5 x 10-20 second hill repeats with several minutes of easy running, or even walking between, to allow for recovery. As an alternative, you can accelerate to landmarks on your normal route. Whichever you use, increase your speed intentionally and make sure to slow your pace adequately between efforts. If you work too hard between repeats, you'll limit the benefit. The key is to finish faster than you start.
While easing into running is wise, cyclists can also benefit from occasionally maximal-effort on foot. There are some workouts, like "running" the Manitou Incline, that are extremely difficult whether you try to do them "easy" or as fast as possible. In such cases you might as well focus on putting forward your best effort and embrace the fact that you'll be sore afterward.
Incorporating new-to-you workouts will often require greater recovery time. Keep this in mind when the soreness hits and remember that the added stress will elicit a positive physiological response from your body. Adjust your training by taking it easy on the following days -- keep moving, but lessen the pressure or lighten load to allow your body to adapt.
Also, be careful about keeping your workouts relatively short while training at a high intensity. Long, intense workouts can lead to injury and immunosuppression, which is best avoided during flu season. Short, intense sessions, combined with moderately-long, lower-intensity workouts, are the way to go.
Intensity and Intervals
I rarely have athletes stick to "base" training exclusively as performing some intensity work during the offseason can help "pull" fitness up from the top by raising the ceiling and allowing more room for growth below. I'm a fan of descending-sets work for this purpose, but you can also select a workout that you'd normally use in-season. The key to successfully incorporating such work in the offseason is to avoid pushing to your absolute maximum.
Try supra threshold (zone 4) sets as follows: 5min, 4min, 3min, and 2min with equal rest to work. Or, if you'd normally do 5x5 as hard supra threshold workout during your competitive season, option for 2x5 or 4x3min now to visit the quality, but not forcing yourself to tap into the psychological focus.
A few short interval sessions can have a big effect while still keeping things fresh. Something as simple, and painful, as a couple VO2 workouts can improve fitness despite limited training time. And as Einstein alluded, there is genius in simplicity. Workouts don't need to be sexy or complex to be effective. However, doing the same workout with the same training load repeatedly, will lead to diminished results over time.
If you normally do threshold workouts at a steady pace, try adding undulations or even breaking your VO2 efforts into HIIT with limited rest. For example, turn your 2x20 session into a 3x12 min undulation where each 2min is just below and each 1min is just above what you'd sustain for the 20min effort. Or if you do 4-6min VO2max efforts try 6-9min of 30sec hard: 15sec easy. This will result in similar if not greater time at the goal intensity.
Stay Fresh with Less Stress
Go with the flow a bit more to avoid offseason burnout. Embracing training variety can keep you mentally and physically fresher throughout the year. You may lose a bit of your peak race pace or threshold power, but this is to be expected during the offseason. Trying to maintain peak fitness throughout the year doesn't allow periods of rejuvenation, nor does it contribute to great performance. Opting for training modes away from your primary sport several times per week while revisiting primary sport every 3-4 days will keep your fitness higher than you'd expect.
Note: If you are new to endurance sports or were not highly trained prior, an "off-season" is often over-rated, but you can still benefit from changing up your training.