I usually do some hill repeats - once a week intervals and hill repeats on alternate weeks and then try and add as many hills into medium and long runs as I can. Some gym work can strengthen legs which can help with the downhills as well.
No refunds, paying upfront no better. GFA is guaranteed - not guaranteed they won't move the goalposts again so best to get well within. Lots of other lovely UK marathons . did London Sunday and must admit it was a great atmosphere and ran a GFA so I will go back.
If you're inspired then go for it. If you have already managed a HM with no problem another year or so of consistent training and you can certainly finish a marathon if time is not the main aim. That inspiration will see you through the training. you will be v lucky to get in via ballot first time so look at having a backup if you want to do one next year.
i dived in at the deep end at VLM 2010 (not recommended!) my running career started with training for that - got round in 4:43. Returned on Sunday to run 3:07.