The mountain hikes will be a good warm up but unfortunately most of them still have snow and will for a few months yet. Some good hikes to get some elevation gain right now would be:
The Skyline Trail. It'd be good to hike up a portion of the way now and then hike back down. I would suggest you could hike up to Sherwood Forest (a section that is covered with trees) and then come back down. The desert weather is nice right now and you'd get a good feel for the lower section.
The Boo Hoff/Guadalupe Trail in La Quinta. This trail has about 5,000 gain if you do the whole thing and finish at the Sugarloaf Cafe on Hwy 74 but it also one you can hike up and come back down.
The West Fork/Jo Pond Trail in the Indian Canyons. To do this entire trail requires a drop off and car shuttle. The drop off is needed because you can't leave a car overnight in the Indian Canyons and the shuttle is needed because you finish in Garner Valley. This, of course, can be done, like the others as an out and back hike but will give you a good taste of some elevation gain.
Another fun desert hike is to climb Murray Peak in Palm Springs. It's not a ton of elevation gain (maybe 1700 ft.) but a good hike nonetheless.
And a mountain hike closer to home is the Marion Mountain Trail in Idyllwild but I don't know if the road to the trailhead will be open in time to do it before Skyline.
Hal
