How can I keep in shape for cycling while not cycling?

My question is how can I keep in shape for cycling specifically while not cycling?
I am nearing the start of my last year of high school (Thank the Lord!
) and this means for the next few months cross country season will begin. I am going to start focusing more of my overall fitness on running for around three months and I was wondering how during this time I could stay fit for cycling while not actually cycling. Will supplementing in running actually keep me about at the level of fitness on the bike when I stopped or will all the running actually destroy what I have built up in cycling over the last few months from not training the same category of muscles.
Also, I am not sure at the moment if I am going to do this or not but I was also wondering if commuting to school each day (10 miles one way and then back in the afternoon) would also help me stay in shape? However, my real question is whether this would actually be a little too much in your opinion? Do you think 20 miles everyday for 5 days a week while running around 6 miles avg. each day in-between be a little too taxing on my body? Or do you think it would be an appropriate amount?
Thank you
Josh M. Holloway
I am nearing the start of my last year of high school (Thank the Lord!

Also, I am not sure at the moment if I am going to do this or not but I was also wondering if commuting to school each day (10 miles one way and then back in the afternoon) would also help me stay in shape? However, my real question is whether this would actually be a little too much in your opinion? Do you think 20 miles everyday for 5 days a week while running around 6 miles avg. each day in-between be a little too taxing on my body? Or do you think it would be an appropriate amount?
Thank you
Josh M. Holloway
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If however you find you don't have the time / energy, you won't lose any cardio fitness by just running instead, and I doubt your cycling-specific muscles will suffer much either in just 3 months. I'd still suggest going out on the bike at least once a week; cross-training is highly recommended.
I do weights (mostly bodyweight and lightish but high rep stuff) too which I'm convinced helps avoid injuries in other sports if done properly.
By the way, I am going to try to hit up the gym a lot more during my off season and I was wondering what weight machine I should focus on. Should I put most of my workout into just hamstring curls and calf raisers or should I add in some upper body too?