While a tab is fine for a compound due to reduced holding weight, I think you'd be better off with a good three fingered leather glove for the longbow.
Make sure your arrows are tuned to your weight/length. You're pissing in the wind right out of the gate if they aren't. A good tune can get you shooting groups with bare shafts at 20 yards.
Start close like in the link, and work your way back. Quit at the first sign of fatigue, focus, etc, and wait several hours or till the next day. A few arrows at a time, several times a day is a better approach. Bend at the knees and back, don't stand straight up like an olympic shooter. Push and pull at the same time.
It'll take till fall to get to where you need to be, where instinct is 2nd nature, and yardage is forgotten.
