Not to harp man, but I saw about 30-40 different bikes that would last longer then the wally world mongoose on cl just last week. And if you want to combat the goat heads on the pavement and grassy/dirt areas consider switching to tubeless and just keep a spare tire on you. The amount of flats I've gotten on my road bike I can count on one hand in 2 years, and the same goes for my mtb with it getting a majority of its flats from pinch flatting on some DH sections on trail 100.
Since you already have the mongoose not much you can do now, but I would say stay away from any solid core tubes unless you fancy a rough ride. Once your ready or feel the need to turn to Craigslist, check east valley/phoenix instead of the west side and you should be able to find some nice bikes. Also if you are in Tempe, go to bike saviors, you can pick up a bike for $50 and build it up in their bike shop if you have the time.
Get a good water bottle and cage, and stay hydrated, since your riding that heavy beast of a bike you may want to look into a camelpak as well. I ride during the summer and commute, tour, and trail ride the big trick is to start riding now and keep doing it so you can acclimate.