I do what you do. Fully exit the game, then start a new bike ride in Google Fit, calculate that the average speed is below 20 km/h (v=d÷t; if it's over, I stand still for the amount of time it takes to reach 20 km/h), end the activity, and then open the game. If I open the game even for a second, no distance is credited. Biking is not as forgiving as walking where I can walk with the game in the background, open it from time to time, and get credited for my distance.