While some of us try to use this verse as an excuse to be out of shape, it is important to not allow temporal things (like exercise) take up so much time in our day that we don't have any left for eternal things (devotions, soul winning). There are however some neat parallels we can draw between physical exercise and spiritual exercise.
I've been recently on a health kick, trying to get back to the fitness level I was in the Marines. I'm realizing it's all about building habits. Eating right, working out, getting enough sleep; they are all habits that are built over time until they are second nature. The same holds true for our spiritual fitness. The more we do it and force ourselves to be faithful, the more we will form good spiritual habits.
If you struggle with good spiritual habits, try keeping a journal and writing down what you did every day for your devotions. Another idea would be to get a calendar and cross off every day that you successfully completed your tasks. Give yourself some accountability with a partner and encourage each other to be faithful. Keeping a good relationship with the Lord takes effort, but it will be rewarded.