Should a 16 year old girl be able to do p90x?

I'm 5'3 110 pounds and have a very slim build (if that matters?) and I'm in shape, Im a runner,so far I can run about 3 miles straight through even longer if I take walk breaks and I do sets of sit ups and push ups at night. I eat healthy but diet coke and sweet tea are my weakness. But I've been trying to do the p90x workout system for about a month now and I can't do a whole workout straight through with out my arms giving out on me. It's driving me crazy why can I do everything else and not this?