I did this ride twice (the last time in '01 I think), one-way to Old Faithful and then shuttling back, and I really lucked out with the weather both times -- quite cold but sunny & dry. It is an absolutely beautiful ride under those circumstances!

So pray to the weather gods! Some sections can get a little scary with the traffic because the roads get quite narrow. Don't let the hill scare you, it's very doable. My parents (in their 60's) did it in '01 and they lived at sea level!