do i have enough weeks to train???

hi all!
i just discovered this site today while i was at work (yes, working hard) and i need some tri-training advice. i have been a runner for years and have completed one marathon and several shorter races. at present, i am running only 20-25 miles per week. i swam competitively in college (10 years ago) but occasionally jump into the pool to train from time to time. my weakest leg of a tri would probably be the bike as i have little to no experience in that area (although i do have a really great racing bike that i have never used). my question is this, could i safely and effectively train for a 1/2 ironman being held mid-september (13 weeks of training)?? i would like to do 1-2 sprint distance tri's prior to the big event as well. i was actually training for the same 1/2 ironman last year but was unable to complete my training and compete due to a knee injury. any thoughts or advice? also, anyone know of any good websites that might contain a good 12-week 1/2 ironman training program? thanks!!!!