Last year I prep'd really hard for fitness as I knew it would be better than it's ever been regarding it! I trained on my own for months before also doing a Manchester 10k run and also some really physically and mentally draining challenges (UK National 3 peaks challenge). When the actual competition got announced last year I wanted to make sure I was training right, hired a personal trainer and he pushed me to the limit (Slightly beyond in a few cases nearly fainted twice in 2 sessions)! Now to be honest that helped me more than anything, but I'd be careful about overtraining, there was a point in my training where I got a slight injury which effected my training for 1-2 weeks. That was a knee/muscle injury and so training had to move to non impact with also injury treatment.
Fortunately that did pay off as the UK National final as I think it was 3rd to 7th was separated by 1/2 points or something like that which is literally like 1 level on the bleep test (I'm not sure of the actual scoring system but trust me it's close).
I have seen a few people training for GT Academy already and I've had a few questions regarding it to. If you really want this there's quite a few people not pushing themselves fitness wise. Doing a set number of pressups or situps is pretty useless that extra 1% you give at the end is where the reward is for getting stronger or running faster so rounding to 10/20/30 is really really useless. That number on the last set should be as high as you can possibly go, you should be collapsing on that final pressup, not counting 30 and saying done... O and i did say set so you should be doing more than 1 set of anything
!
Anyway I'm not saying get a personal trainer, what I will say is that it's very important to train right, just exercising in most cases won't cut it! You have to be pushing yourself so that when you exercise saying 1 more after you've already said 1 more 3 times already is still happening (This is also how you can push yourself mentally, as usually your body is saying give up but you can do more!).