First, Nick..thanks for the nice words!
~50 +/- miles sounds like your intended goal?
So much to choose from.
There's this classic loop of course:
http://www.backpacker.com/destinations/hikes/399192 (closer to 50 according to most people) with some minor scrambling (class 3 perhaps), and IMO, easy route off-trail route finding. Get to do perhaps the most iconic 14er in CO (other than Pikes Peaks)
View in the Snowbank Lakes area:
Your climbing won't be this technical :) , but does give an idea of the views from the general area of the Longs Peak summit ( I dabble in climbing..mainly because my friends know what they are doing! ;) )
The loop above does hit the highlights, but I prefer the Mummy Ranger personally. Less people (relatively speaking; it is a national park :) ), very beautiful and just aesthetically pleasing.
Start at the Lawn Lake TH, work your way off trail to Mt. Chiquita (there is a use trail by Chimpmunk Lake heading up the ridge), then to Fairchild, drop to the Saddle, up to Rowe (or Hagues then Rowe if feeling ambitious) work your way to Stormy Peaks Trail down to north boundary trail to Cow Creek, west along Black Dark Mtn Tr to Lawn Lake and back. A LOT of cross country travel... Done this loop in different pieces over the years (including the cross country scrambling) as day hikes or backpacking loops. The off-trail scrambling will make up for your mileage goals. Obviously I did not lay this out as much as the Backpacker Mag loop above. You may want to print out appropriate 7.5 topos for the Mummy Range scrambling (class 2-3; easier than the Longs Peak approach above).
View for near Ypsilon:
Looking back to Ypsilon (notice the "Y" coliour)
Again, I love this range:
In fact, the best part of RMNP is off-trail IMO. The mountains are relatively gentle despite their height and have plenty of places for off-trail fun.
If you want to stick to on trail delights, may be a little more difficult in terms of mileage goals.
You could easily make a variation of this classic loop to add mileage:
But the views are rather lovely:
I liked it so much, I did this loop for my Bachelor party. :)
Now, the weather in RMNP depends on the elevation, but generally speaking, I think high 60s/low 70s in the day and high 30s/low 40s at night is about right for the higher (10k+ ft) elevations in summer.
Finally, please be aware you do need a bear canister in most instances at RMNP now:
If you have a cross-country campsite, not needed. (My buddy and I did not need one last summer when we did an overnight climb and bivied below Notch Top)
Other questions? Shoot them my way!