Training is dragging

At the moment my training is dragging a bit, at least compared to previous years. I think the main problem is that am trying to train like I did when I was better conditioned and am trying to do too much in a short time. I am also a couple years older, so need to make adjustments for this as well. My next planned race is a 6-hour benefits run in March, which means I have around 7 weeks to train. I think the next couple weeks I am going to try concentrate on core training, than gradually increase the mileage up to the race. My goal for the 6-hour race is 50 km, but anything over the marathon distance is acceptable.