I'm 6'3 and 185lbs and my longboard is about the same as yours (slightly narrower and thinner - so proportional to weight probably quite similar) and it seems to float me pretty well. So (as usual!) I agree with jaffa, the board should be fine. I'm assuming there are no faults with your board.
Remember that paddling is an 'unnatural' activity - you dont get paddle fit or use your paddle muscles doing day to day activity (other than swimming or certain weight lifting). So its not unexpected that after years of not surfing your paddle muscles are both weaker than you think and tire quite quickly.
But - that said - I noticed this review http://custom.surfboardshack.com/90-pea ... formula-1/
which suggested paddling is a bit hard with this board. It doesnt seem to have excessive rocker or anything, so I'm not sure why its a problem. The surftech site http://surf.surftech.com/featured-board ... formula-1/
says the volume of the board is 66L, which is slightly under but pretty close to the volume of most 9ft boards (around 70-72L is fairly standard); and in any event 66L should be fine to float you.
I would keep going until your fitness has built up a bit and if you are still frustrated, then maybe look at a different board. Alternatively, if you are able to borrow a different board maybe see if it makes a big difference and then make the call. But really your current board should be fine - most likely a 'user problem'.