Get as fit as possible - a) it will make it easier for yourself and more enjoyable, and b) to lessen the chance of any altitude related illnesses. We very nearly lost one of our team to cerebal oedema - a Gammow bag saved the day. The climb is not technically difficult but it's a high peak and it's a long way to the top - the only realyy steep bit is the last 100 feet to the top. The summit day is a long day, especially if you go to the summit from a high camp and then back down to a lower camp. You need to watch the weather patterns closely. When we did it, high winds were hitting the summit at about 12 O'clock - you could see a plume of snow blowing off the summit - so make sure that you are up and down before that happens. Also, there can be lots of hidden crevasses on the route so you are better walking across those when they are still frozen. The whole trip to Mera is awesome - the walk in is superb and the view of Everest from the summit, is worth all the effort.
Spent 2 weeks in Joshua Tree climbing and hiking (park permit only allows a 2 week stay). We went end of October - daytime temps are in 70/80's very pleasent (but it did snow one day), night time temps below freezing. It gets dark dead early so it's worthwhile stocking up with fire wood (not from within the park). Camped at Hidden Valley campground - a great place to camp but you need to get there early to make sure you get a pitch - you may have to hang around waiting for someone to move and then jump in the spot quickly! The park rangers are very strict on the numbers of camping spots so if you camp in a spot that is'nt allocated - they'll move you on. The climbing is brilliant - excellent bouldering around the campground and awesome routes of all grades. There's loads of old mining trails about - so the biking should be good - but I'd check first - there is so many regulations in the park that you may find yourself in breach. Theres no drinking water in the park so you need to stock up in Joshua Tree town (20 miles fromm campground)- theres a tap outside of the climbing shop on the corner of the road up to the park. JT is a great place - enjoy