Week 4...all over again

So I think I will be doing week 4 a second time. I did another workout of week 4 tonight because i tried the first week 5 workout and I just couldn't do it. I didn't quite finish the end of the workout i did tonight. I did almost everything. I just couldn't make it through the last 5 minute run. But i still feel great and I know I got a great workout. I will be doing another week 4 workout on Friday and I'll go from there. I am getting worried that I will just not be able to finish this program. Is it possible that I'm just not cut out for running? Well, I'm going to keep trying and I guess we'll find out!