That's what I'm going to do too.
67 points in Destruction currently and it's very slow levelling up.
However, Wall O' Flame...
It takes a while, yeah - I've had the Mage sign on since the beginning of the game, it comes from the set of stones right by where you start and seems to help a bit. I've used destruction as my primary damage dealing throughout the game (fire for early ranks, shock for when I've got more magicka to spare) and there doesn't seem to be any easy way to level it besides the long grind. A lot of people seem to be suggesting summoning atronarchs and destroying them is the best way to do it, but the skill progression is very slow this way... maybe because I'm in the early 70s for destruction and can only get hold of a frost atronarch for conjuration. Plus, since the atronarchs never actually become hostile to you, you never really get tagged as being "in combat" and your conjuration doesn't increase.
I think the comparative mob level plays a big part in how fast your destruction levels up - I know I've gained at least a level in the skill for each of the last two dragons I've fought (ancients, the frost dragon before that went down in about 10 fireballs

), so I may just hunt a few more down to get to skill 75. Aside form that, clear outa few random caves or castles using dual-cast fireball and you should be laughing. Just stay away from mage places - For the last 5 levels at least (I'm 31 now) mages seem to love dual-wielding the expert level shock spell and expert ward, which really sucks when you're trying to cast your way through the skills

.
Next stage is to buy up all the petty soul gems I can find, fill em with rats and spam waterbreathing rings for fun and profit and enchanting skill. My intention is to get at least 4 resist magicka enchants at 25% and 4 fortify destruction 25% on my gear for free death bolts and immunity to magic. That should do the trick
I bought them from Faralda in the College, when she eventually stocked them.
I believe there is some level scaling, which is annoying. And I'm level 31 now. So it could be every ten levels? Pure speculation however.
I think it's dependent on your skill level. I first saw adept spells crop up when my skill hit 50 and I've just managed to purchase expert level spells after hitting 70, though I was checking more often for the latter so the former may have been available earlier. I was lv29 or so when I got expert spells though, so I don't think it's only char level that decides it.