The net altitude drop from the Rush Creek area to Red's Meadow is not substantial, but the terrain between the two points is gonna work you. From the north, the climbs and descents grow increasingly extreme, beginning with the relatively gentle Island Pass (what a view!) and culminating in the long descent to and subsequent toil out of the Shadow Lake area. Non-purists might feel inclined to skirt the northern shore of the lake and drop down to the PCT en route to Red's Meadow rather than hike the intense switchbacks past Rosalie and Gladys Lakes to the next rise, but we would advise against it. Roslie and Gladys are beautiful, and the descent past Trinity Lakes is a nice one. It would be a shame to miss these. Past Trinity the trail drops a seemingly interminable and dusty stretch to the populated Devil's Postpile National Monument. Follow the signs closely. it's easy to get sidetracked in here.
By far the toughest section facing the north-bound hiker is the ascent out of Shadow Lake. The initial stretch is attractive and easy, if popular, but the next part, after the trail diverges from the Ediza Lake Trail, snakes often steeply upwards toward the pass-like cleft leading to Garnet Lake. Make sure you have plenty of water for this section, particularly late season, where the only water to be had is just past the crest of the "pass" at a small tarn. After this ascent, it's all about Banner and Ritter. If you're coming from Silver Pass, you've been staring at these monsters for days. It's a pretty spectacular stretch from here to Island Pass. The climb up from Garnet is easier, but can be a travail late in the day, but it drops you down to Ruby Lake, one of my favorite spots on the JMT.