I assume you are riding your Marline 6 on roads or MUPS? Put some slicks on the wheelset if you have not already and lockout the fork. That should help and get some bar ends for it to give yourself more hand positions. The Marlin is a heavy bike 35 lbs and you will never be able to keep up with fit riders on proper road bikes but it is a good beginning for you. I started on a Marlin 6 in 2014 and only averaged 13 mph to start.