OP if you are in your 30s and fit and healthy you shouldn't need viagra.
For me I don't masturbate a couple of days before hand, book an hours GFE session which will involve lots of foreplay. Once rock hard start penetration, but make sure you pull out of you feel your going to cum early or not feeling it, keep switching positions, eventually you'll get a rhythm and ejaculate.
Most of all, just empty your head and relax. It took me 3 punts with my current SP before I climaxed inside her (in a condom of course).
But with a good 45 minute build up you won't feel hard done by if you only cum once.