I don't have any trouble with motivation. It is still cool in the morning or at least not hot. In fact, this morning I rode 32 miles just because it was nice at 82f -- regular route is 10 miles.

So it is only the afternoon that gets bad. I just take it easy and make sure to stay hydrated.

As for motivation, it has been 540 days since I last drove my car to work (and that was one day). I don't intend to change away from my bike for any reason.