Position is the biggest difference to TT times
Specific fitness is the the biggest difference to TT times.
so I’d put in a few weeks of specific threshold training, 3 or 4 days a week, an hour a session, see what your times are like at the end of the season, then if you’re still hooked look at buying a cheap second hand TT bike over the winter. there are some real bargains to be had, as the OCD Triathletes offload their year old kit for peanuts.
after that, its position, look at getting a flat back and try to move forward on the bike a bit, aero bars will obviously let you achieve that, then think about helmet, wheels, skinsuit etc.
I’m still on a road bike with aero bars. best time this year is 22.34 for a 10, 57.13 for a 25. taking the plunge this winter for a full on TT bike. It’s addictive. also, a bit of weight loss with the training increases your speed more than anything else. I started the year at 13.75Stone, now I’m 11.75, and you can fit a skinsuit 🙂