How do you know when to give your muscles a break?

bitesizedonut
I thought I remember reading some where that crunches or core exercises were the one type of exercise that you could do without detriment - has any one else heard this? Also, should you take a break from resistance work outs every so often (even if you alternate upper body and lower body groups every other day)?

I've also had some one tell me that I shouldn't work on resistance exercise and aerobics at the same time as it isn't an efficient use of my energy stores (in other words, I won't be able to put as much into either activity if I do them back to back)?

I also want to get into a good running routine. Should I run daily? Every other day? I prefer long distance running and when I did x-country competitively our coach had us run every day but I feel I don't always have the time for an every day run - how many days can I skip without losing my improvements in time and distance?

Any advice?
How do you know when to give your muscles a break?
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