with hills weight is the most important factor- you need a good level of fitness as well but fitness only takes you so far- the amount of extra effort and energy to haul 235 lb up a hill compared with e.g. 180 is huge. The amount of extra training you would have to do match a 180 lb person climbing a hill is very difficult to achieve.

But 400 miles is not a lot. As long as you are not competing there is no need to rush and compare you performance to others- just take your time. Change down to a low gear early and just keep going up the hill at a sustainable speed. A heart monitor might help.