You will find bicycling options of all lengths and difficulty.
The easiest is along the Boise River Greenbelt. Ironically, it is right at your 25 mile goal.
https://www.ridgetorivers.org/hikes-...in-bike-loops/
For mountain biking, I suggest checking out Boise's Ridge to Rivers Trails:
https://www.ridgetorivers.org/hikes-...in-bike-loops/
Since you will be here in the heat of the summer, you might consider taking your bike (either by riding it or driving it) up to Bogus Basin Ski Resort and enjoy their Gravity Park. You can use their chairlifts to go uphill and cruise as fast as you car to downhill. Rinse and repeat all day!
https://bogusbasin.org/your-mountain...-gravity-park/
I am sure you will find something that will work well.