I'm in just about the same position, but a month out -- (re)started riding on my 10-year-old Specialized Rockhopper in the middle of July (prompted partly by the TdF and partly by my four-year-old who wanted to get a Tag-a-Long).
Most of my rides have been going up 4% to 6% grades on the Salt Lake City "benches", and your description of speed, etc., sounds very similar to what I've been doing.
My advice would be to just keep at it. As opposed to jogging, I find it has been easier to go biking two or three days in a row, even before getting totally in shape. Just keep going, and keep doing the hills.
And after a month, I can definitely take the hills much better (maybe not a lot faster, but definitely more steadily and confidently), and I managed 18 miles in a one-hour flat ride yesterday morning. You should be able to handle a 25-miler in four weeks.