I found a good core routine online which I do 5 times a week, it's about 6 minutes of exercise so really doesn't take much out of the day, sometimes will do two sets. Basically sit ups, then crunches, then leg raises, then leg flutters, then Russian twists then bicycle twist. 45 seconds on, 15 seconds off. Am also aiming for 1,000km of running for the year so have worked up a schedule through the year where I progressed from 8km a week to now doing 28 km a week, will peak at 30k a week in November then ease back to 28km again in December over the holiday season, but I've lost weight and toned up as a result and feel a lot fitter. It's nothing too crazy or extreme but it's made a noticeable difference.