Yes, the trail south from Red's Meadow is mostly uphill, and yes, it can be hot, since Red's Meadow is about the lowest point on the John Muir Trail. But as hills go, it's pretty mild. Just out of Red's Meadow you trudge through the remnants of the Rainbow Fire of a decade past. And then it's an asecent up the world's longest switchback to Crater Meadow and its dodgy log crossing. Southbound hikers traveling at a moderate pace may find Deer Creek to be a good place to crash for the night. The next water after that point is a good six plus miles up the trail at Duck Creek. The views of Silver Pass are tremendous along the ridge high above Cascade Valley. Purple Lake is heavily used. I'd be perfectly happy to never camp there again. Lake Virginia, however, is the nicest of the high lakes by far, nestled in a flat, open High Sierra Basin. Then finally, after miles of uphill, the last portion of this segment drops you like a stone to Tully Hole
The climb out of Tully Hole is warm and steep, the slope made mostly of shale. But Lake Virginia is well worth it. Most of the long approach to Red's Meadow is downhill, and not precariously so, although there is a short steep drop on the final approach to Duck Creek. Remember to stock up on water at Duck Creek, for the next water is about six or so miles down a long ridge at Deer Creek. This segment of the John Muir Trail is more or less a breeze, especially for the northbound traveller who remembers LeConte Canyon.