I haven't done it in several years, but when I was still in the Army myself and a couple of other guys from my unit did the round trip in about 8 1/2 hours. We were all fair (me) or better runners (them) so if you've been running I can't see you having too many issues. When my unit did it their were several guys that ended up with headaches and trouble breathing because of the altitude.
The three miles or so after tree line sounds about right. As mentioned thunderstorms are common. I've done it 3 times and all three trips had some amount of rain.
It would be really hard to get lost. The trail is very well defined from all the foot traffic.
