W1D3 got worse...A LOT worse, what happened?

First post here, so slight background. I'm 26yo male, 6'2" 310lbs. Former military and football player, haven't really exercised in the past 4 yrs I got out of military. Just started working out and eating well again. Heard about C25k and thought I'd try it.

Without changing how I ran or what I am eating/hydrating, Day 3 got worse for me. The first day I made it about 15min into the 20min run/walk. Second day made it about the same. Third? 6 minutes in and I hit a wall. Everything was sore and hurt. And yes I rested, 1 day the first time, 2 days after the second time. I wasn't even out of breath, or had my pulse running. I was just unable to move my legs. I've never really had this happen before, even doing 2 a days with football. Why would it happen the 3rd time, and not the first or 2nd time?

Some additional info before I waste peoples time: I am well aware of running form, have good shoes and general running etiquette. Right as I started to feel it, I was double checking that I was landing properly on my feet and I wasn't leaning forward or backward.