Need help to achieve a 55 to 70 km ride

I got the cheapest 'adult' bike they'd sell at walmart. Did 20 km today and even with an additional gel pad sitting on the seat broke my ass. It's not the bike itself moreso the stupid seat design on bicycles. Any idea what I can do on the cheap to ride longer in about 2 weeks? Looking for cheap hacks like stuffing towel in my pants, wrap plastic bags around seat, wear vaseline smeared diapers. etc..