Wow. That's pretty unusual to have boarding numbers that high when checking in at the 24-hour mark, but obviously it's not impossible. If there are lots of people flying in to your origination point on other SWA flights and transferring to your flight, then they all would have been able to check in for your flight 24 hours before their FIRST flight, which gives them a head start on you. Other than that, the only explanation would be that a lot of people were just that much faster or luckier than you.
In the end, however, it doesn't really matter what the reasons are at this point. You have what you have and that's that. No boarding # on SWA puts you in the "back row" or any specific place, for that matter. It's just your place in line and you can select any seat you want from what's not already occupied when you get on the plane. FWIW, there will be medical pre-boards (usually few, if any), A1-60 (with there frequently being few or no passengers in the A1-A15 slots reserved for Business Select ticket holders and top-tier frequent fliers), families with kids under four (who board between the A and B groups) and B1-29 ahead of you. There will still be B35-B60 and everyone in the C group behind you. In my experience, with B30-34 there still should still be several blocks of seat pairs together and there may or may not be any three-seat blocks (which is an entire row on one side). It is likely that there will be more of these available in the back of the plane. The worst-case scenario I can envision is that one person in your party of five may have to sit alone in a middle seat, but probably can get one right in front of or behind others in your party.