Help! I've been running for 30 minutes 3 times a week for about 3 weeks now, but on Saturday I was completely floored. Felt fine for about 20 minutes then just couldn't carry on. I walked for a bit, then ran again for a few minutes, but had to stop and walk home. My legs felt like lead. What happened. I'm a bit scared to try again today. Any advice? Does this happen to other people?