I think it is valid. I recall that after the season Peyton mentioned he was not 100% there yet in his own recovery. It was a long season and the last game was one of, if not the coldest ever in Denver. For a dude with multiple next surgeries to go for 6 months and gain the ground he did, its not surprising that he started to wear down a bit at the end.
I have an expectation - as I believe does Peyton - that he will be better in overall strength and endurance this year. Hopefully an offseason without surgery will allow some more nerve regeneration and the cold won't be as much of a factor next year.
And we need to focus on a running game to further alleviate these concerns..