It's manageable. We stayed at DLH this weekend and went to the parks on Sunday (9/14). It was definitely hot. Fortunately, we had two factors going for us: 1) we had early magic morning entrance and 2) we were in the parks during September. Since we used early magic morning to get into DCA at 7 am, we were able to go on the following rides before 11 am (when we had to go back to check out of the hotel: Radiator Springs Racers, Toy Story Mania, California Screamin', Silly Symphony Swings, Goofy's Sky School, Gizzly River Run, Wilderness Explorer Trail (finishing all 6 challenges), the Little Mermaid Ride and Soarin'.
Because it was September and because it was super hot, crowd levels were reasonable. After checking out of the hotel and eating lunch in Downtown Disney, we made our way into
Disneyland around 1pm, and saw people streaming out of the park as we were walking in. Most of the rides we went on had only 10 to 15 minute waits, and usually only a 5-10 minute wait until the line got indoors (which is really what mattered when it was that hot). From 1pm to 8pm (when we left), we were able to get onto the following rides: Big Thunder RR, Pirates, Tarzan's Treehouse, the Enchanted Tiki Room, Indiana Jones, Small World, Star Tours, Buzz Lightyear, and the Matterhorn.
You should be aware that the Monorail wasn't working from noon on, mainly because the trains don't work well in the heat. Don't count on it as your way into and out of the parks on very hot days (except for early mornings).