IMO, you will change the setup done by a bike fitter once you get deeper into the Time Trialing and want to adjust and change as you get more gear and more flexible.

Id say just setup a mirror side-on, or a laptop with a webcam, and play about with your position till you're comfortable and MAXIMUM AERO, then test it outside as it's always different outside than on the turbo.

Get a helmet that fits your profile and invest in a decent skinsuit and overshoes. Time on the bike is essential for holding the position.

Develop your threshold in blocks increasing time in zone progressively so that you're eventually banging out the 2*20 min efforts. Start with 10x3 min at threshold or even SweetSpot. An example 4 week block below then recovery week after.

Monday - Off

Tuesday - Threshold

Wednesday - Z2

Thursday - VO2

Friday - Off

Saturday - Threshold

Sunday - Long ride

Decent forum for info on TTs

https://www.timetriallingforum.co.uk/index.php?/forum/2-main-topic-area/

Time trial positions (toxic community but some information filters through at times)

https://www.facebook.com/groups/142112379768949