Hi Skunkpack, just hiked Devil's Slide Trail yesterday, 9/17, to summit of San Jacinto, here are some things you that may help you. For background, I used SJ as my training mt for a successful first time Whitney summit on 8/9. I climbed it 5 times during June and July and I've grown to love the Devil's Slide trail. If you can hike it and feel good at the end, in my opinion, you're physically fit for making Whitney (still need to do the other key things like acclimate, etc.)
Hike up takes about 3.5 hours, hike down takes about 2.5. There is no water anywhere. I take a 100oz backpack and two 16 oz bottles. The hardest part by far is the last .3 miles to the summit where the trail goes away and you have to clamber up and over some large boulders. Watch your step. View is spectacular and actual summit is very small, not like Whitney. Tough again going down to find the main trail again as you descend for .3 miles. Once you find it, you're fine.
Coming from sea level to 10,843 and back down again requires me to take Diamox the previous day. Otherwise, coming down in the afternoon sun, the altitude causes some nasty headaches.
Huge, beautiful views at almost all points and a real treat.
Have fun.