i'm 17 stone and i've started the c25k this morning, i must admit i didn't do the whole 30 mins but i did warm up on the xtrainer for 10 mins and cool down on the bike for 10 mins after, next week i will start it properly. Just watch your knees! and noisy treadmills, i had two this morning - dunno if it's just my size but the big guy on the other one wasn't making a noise :/
Dont be scared of it!!! The walk/run alternaton is way better for preparing you to run than keep fit dvds or walking. You sound eager to get going so go for it!! As the famous sports brand succinctly say "just do it"