So, assuming that you're using ~90% of your battery capacity on your 44 mile rides you'd be averaging ~11.4Wh/mile. To get to your 70 mile goal you'd need 70x11.4=~800Wh total battery capacity. Your current battery has 48x11.6=557Wh so you'd need an additional 800-557=243Wh battery. For a 48V battery this is the same as a 5Ah battery, about 1/2 the capacity you1 have now.
Google brings up electrictrike.com when I search for Sun Seeker. Is this the correct bike? If so, they offer additional battery packs starting at 48V 10ah. I'd suggest going with that.