Don't be afraid to lean water starting on the long board, that's the way I did it. Occasionally, you will fall to the windward side of the board with the sail still flying (try to keep it flying). From this position, you are ready to try a water start. If there is enough wind (about 12-15 knots for you with a 7.5 - 8.5), give it a try. No special effort has to be made, just take advantage of the occasional opportunity. Before long, you will nail one.
Most windsurfers don't try to learn waterstarting until they have a smaller board, but there is no reason to wait.
Roger's suggestions are right on. Railing the board while pointing upwind is a little more advanced, but it will allow you to gain 5-15 degrees closer to the wind. It's not big deal while free sailing, but it's a HUGE deal if racing or trying to get upwind as fast as possible.