700x28c tires should be good to handle gravel bike paths, and will give you a bit more cushioning from bumps on the road. Will your bike fit them? Also try running at lower tire pressure. A sport touring bike will combine features of a road bike (and easily add fenders, rack etc) but trades off some performance for more durability. I think the sport tourers as opposed to classic touring bikes tend to be lighter weight so zippier to ride unloaded, and have higher gearing, while perhaps not being quite as well suited to very heavy loads.