You just started riding a few weeks ago. You need to cut yourself a break. Doesn’t matter how fit you are in other disciplines. The fact that you are fit just means the gains will come quicker as long as you stick with it. Your 25mph top end is probably about right. Average speeds solo are very different than group/bunch riding where you are in a paceline and benefiting from the draft. You’ve heard of the Richter Scale? Once you start averaging over 20mph solo, each mph above that is just what that’s like effort wise.