Could be any number of things: socks, shoe fit, insole type, cleat placement/type, tightness and location of the shoe straps, or even position on the bike. Most bike shoes shouldn't wear out after only 18 months, so I doubt it is the age of the shoe causing the problem.

In order, I would try these things:

1. If you have a multi-strap shoe, loosen the front strap a bit. Feet do swell over time and in hot weather, so that could solve the issue.
2. Try thinner socks or no socks.
3. If you still have the original insole in there, go to a reputable bike store, describe the problem, and see if they can recommend a good insole.

If those options don't work, we move to more expensive territory:

4. Pedal/cleat type: Are you on road pedals or MTB-pedals? MTB pedals come with smaller cleats and, in my experience, it can be easier to get numbness on long road rides using those.
5. If you think your pedal type is correct for road riding, consider getting a cleat fitting from the LBS. Explain your issue to the fitter and make sure he understands that's why you're there.
6. If nothing else solves the problem, look for new shoes.

either the shoe is too tight;
or, the footbed is not stiff enough and transfers the flex at the pedal into your foot after a while

although it's odd that this is only a recent thing
have you changed anything recently?

the only thing I changed is the distance (and the heat). I have gone the distance before, but not in that heat.