mmmm. Cake!
mmmm. Beer!
mmmm. Marathons!
Okay, so I'm supposed to provide advice. Well, welcome Warbs and Scandi P. Do it! Don't do it! Drink the beer!
Halves in 1:45 to 1:50, yeah, should get round in under 4:30, but it really does depend on how many miles a week you can put in. The year I ran 1:46 (as a young'un) I only ran 5:06 or so in London because I didn't train and hit the wall at halfway.
If you can get 30 miles a week before starting a proper training plan, then great; as far as plans are concerned, I don't use them any more... just try and do one efforts (week 1 flat, week 2 hills), one longish tempo run (6-8 miles at half marathon pace), and one v long LSR (build from 10 to 20 and try and keep regular 15+ runs for about 6-8 weeks). If you can run more days, keep them as 3-5 mile recovery or easy runs. I'd love to do 10 weeks at 50 miles per week leading up to the race!
For anecdotal advice, one of my clubmates has just spent 6 months running between 24 and 29 miles every Sunday, aiming for 250 per month (!!!!) and has just taken 10 minutes off his PB (set last October), with a 2:59. 2 years ago e bottled every race and couldn't beat 3:30...!