How long of a hike is it from H.M. to cotton wood lakes ?
about 6 mi, depending upon where in the basin you go
Cotton wood lakes to the top of Mt. Langley ?
about 6 miles, depending upon where you camp and start from
Should we take new or old army pass?
Different experiences, I prefer old, it is about a thousand feet less of gain, up and back. Also nice to go up one, down the other.
What is the difficulty of the hike? We did Mt. Whitney three weeks ago on a one day permit and made it.
This is easier, there is less elevation gain, you start from about 10k, instead of 8k.
Is there good fishing in the cotton wood lakes ?
Not my experience, had to work to catch them, there are special rules.
What lake should we camp at.
Depends on your itinerary.
If you are going up and down NAP, High Lake (desolate rocky area) or Long Lake (in trees, but .7 mi further down)
If you are going up and down OAP, there is a bluff area to the east of Lake #5 (the one below OAP), where there are lots of flat areas. You go to bed and wake up looking at the pass-neat)
If you are going to go up NAP, and down OAP, you'd want to camp in the wooded area to the east of Cottonwood lake #4 (the long one), one of the nicest areas to camp in the basin, I think. It adds 1.5 miles to the climb up NAP as opposed to Long Lake, but you will walk right by the camp on the way down OAP.
Please use rigorous leave-no-trace technic--this whole basin is very fragile and subject to damage.