At 56 it will probably take you a couple of years to approach the group ride speed. Hang in there and try to ride with them whenever you can. Don't feel bad about getting dropped. Once you can do 15 mph or so, then you might try 3x3 intervals at your LT once a week (3 minutes on 3 minutes off). If you can build up to 4 or 5 of these once a week, it will help your speed a lot.

Frank