Anyway, this one was described as "moderately difficult / a good cardio workout / not for beginners". Yes, yes, and yes — although maybe shading up from "moderate" towards "strenuous". It's a steady uphill pull from the trailhead up Mitchell Rock Trail to Twin Peaks, steepening on the Eagle Peak Trail up to Eagle Peak. It's quite possibly the toughest climb I've done so far. On the way down, we take Eagle Peak Trail down to meet up with the Back Creek Trail.
But it's worth it: great views, once you've recovered enough breath to appreciate them; lots of hawks soaring around and above the rocks; and lots and lots of wildflowers. In particular, on the way down the trail passes through a slope of wild sage, all of which is in flower at the moment.
There isn't decent trail information available online for Mount Diablo: the National Parks leaflet has a very simplified map, but not enough detail to plan a hike or work out where you've been. Slightly annoying, as the EBRPD parks all have good trail leaflets available free at the park trailheads and online. But I can't complain too much: the official trail map is published by MDIA, a volunteer organisation who do a lot to maintain the park. I really should get around to buying a copy.