Good idea, but rural players won't like the idea of having to pay for this feature.
Since rural players (taking your example of 25km away from a gym) are far from a gym, they can't earn coins from defending gyms, therefore they would have to buy them with real money.
Paying real money for every time you want to use this feature, and for it to only last for 30 minutes? This would cause an outrage with rural players! Trying to get your 7-day streaks would take a toll on your wallet!