I say go for it! As long as you're basically healthy, you should be fine as long as you're happy to take it easy, train sensibly, and not plan for a really fast race. Don't worry about the extra weight as it will come off gradually as you get into the training. Are you sure you're very overweight at all, as by my reckoning 245 lbs is only 17.5 stone, which doesn't sound like an awful lot if you're 6'4". Even if you typo'd and are actually 345 lbs, that's not too outlandish for your height.
To put the height/weight thing into perspective, my chubby boyfriend is 5'8" and hovers around 18-19 stone (not all fat, he's a ridiculously strong ex-rugby player but with a build totally unsuited to running!) and he managed to trundle grumblingly round a 10K in 1hr20 with minimal training.So if you're willing to put the training in, you should do great!