Quote Originally Posted by chick on a bike
Although I did not know that overtraining could affect the immune system by making it weaker.....How does it do that??? ~scratches head in wonder~ I know it can irritate my knees! *lol* ^^ :P
I am no sports medic but I suggest you check this issue out in good sports medicine books, maybe especially those about cycling (I've seen a couple cited on TE but can't remember the titles). Discussing this with a friendly doctor could be an option too. Generally they worry about people being inactive, but being overactive can, in extreme cases, become problematic too.

Doing anything in EXCESS can weaken your system: if you use all your energy to get better at one thing (usually: going faster) you may not have much left to fight simple infections, for example. There are a couple of threads about overtraining and/or sickness you could read from this discussion board, too. Just make sure you listen to your body and only increase your training hours/efforts gradually (10% per week is generally recommended). From what you tell us - 4 times a week, 15-20 miles - I don't think it will be an issue for you... but cycling can become addictive!!! so who knows?

Keep riding and don't worry, I'm sure you'll have tons of fun!