I discuss getting outside and some options in this video: HERE
You can learn about common issues with low heart rate in the cold HERE
To Train outside the key is to have a few sport options. None of them really need to be done with great skill, although overtime you will become more proficient at these activities. All sports must be gradually taken on however, do not let your cycling fitness mislead you to think your muscles and connective tissue are ready for running.
Great options for cross-training (indoors or outside) are running, pole-running (Nordic-walking), strength training, cross-country skiing, snow-shoe, ruck-hikes, rowing, swimming, snow-shoveling and even fat-biking (or riding your 'normal' mtb) are all great options. Remember you can combine the above sports with each other and also with some time on the indoor-trainer.
The point of these activities is to:
a) not injure you (again, ease into running with 1 min run : 1 min walk/hike)
b) provide a mixed stimulus to boost your fitness (e.g. skiing challenging heart)
c) provide a mental break from main sport (and indoor monotony)
d) get you outside in nature (why many of us started / continue in our sport)
e) see your friends! (this is very important)
So take your low-intensity days and challenge yourself to try something new. I recommend grabbing some poles and going for a hike/run in your local hilly forest. Start with about 45 minutes, leave knowing you could have done lots more and resort to hiking at first sign of pain/soreness.
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