As long as the horse is working for an hour 5 times a week, and some of that includes hacking with hills with some longer canters you will be absolutely fine. His jumping muscles will also need to be up to it, so jumping once or twice a week for 4 weeks before the event should see you both being alright!
Agree with Tigers-eye. Be careful not to go overboard you don't need them particularly fit at Intro Level a good level of daily work is plenty you certainly shouldn't need to do serious canter work. If you get a horse too fit it can cause more problems with them getting to settle.
Just hacking and schooling 5/6 times a week is plenty for Intro, as long as it's proper work. You won't need to do any specific canter work.
I would start preparing 6 weeks before your first event. That would include having some lessons, getting out on some long hacks with a few canters for fun, schooling 3 times a week, jumping once or twice a week, getting out to some dressage and show jumping competitions, and some hunter trials and/or cross country training.
Lots of confidence boosting stuff, and if you do all that, you'll both be plenty fit enough!