Riding is the only thing that really makes riding any easier. The Gym and circuits may be helpful w/posture, overall muscle balance, and looking good at the beach, but unfortunately do diddly for the bike, absolutely nothing. You mentioned you have started to notice a change in muscle tone, that's good but you are still only in the very begining stages of getting in shape.
If you've only been riding a month have some patience. It can take 3 or 4 months to get some basic bike fitness built up, especially if you haven't been in "riding" shape before. Folks who have had a high level of fitness can usually get back more quickly - "muscle memory" is the term many like to use.
Age can be an issue but in our 40's we are not over the hill yet. At 44 I manage to race for fun, but I am more achy after tough workouts than when I was younger, and it takes longer to get in shape than it used to.
Consistency and moderation in training are the key. Don't pedal with too much effort (use your gears), try to ride 4 days a week (substantially more fitness is gained with 4 days instead of 3). Build up your mileage slowly by adding a couple miles each week, but try to get to a point where you can ride for 60-90 minutes at a time in the next few weeks. In the months leading up to the trip you should be doing at least 5 or 6 hours a week on the bike.
If you are getting off the bike during your ride, do you just not have any easier gears? You might be starting the hill pedalling too hard. On the flat, and using the right gears/effort, riding a bike can be made to happen so it doesn't take much more effort than fast walking/jogging. If you can walk for an hour, you should be able to ride for an hour.
Loosing a little weight can go a long way to making the riding easier, especially uphill. One stone will make a drastic difference. Once you are able to ride for more than an hour at a time, loosing the weight will be a little easier. It's a bit of a catch 22 at the moment. You don't have the fitness to ride for an hour, but you need to ride for more than an hour to start loosing the weight. Take it slowly, make a commitment to yourself to do this thing, and then do it. October is a long way away. But while sharp whit and gifted intelligence allowed us to miss our homework in school sometimes, your enjoyment on this ride is dependant not only on doing all your homework, but learning to love doing your homework. As you get in better shape, doing homework (riding) will become more and more fun.
As far as diet, we all secretly know the answer regarding our own circumstances. A well balanced diet is important, but portion control is another thing. Making our bodies change physically (adapting) requires some degree of discomfort along the way. A little discomfort on the bike when training, a little discomfort (in our heads and bellies) when contemplating that second trip to the buffet, when going for that second slice of pizza, when contemplating grilled chicken or fried chicken with some delicious creamy blue cheese sauce - you get the idea. I had a chocolate shake with my cheeseburger yesterday instead of the usual coke. My pants feel a bit snugger today, nothing the next few days of riding won't handle... as long as I don't have another chocolate milkshake again today. /img/vbsmilies/smilies/wink.gif