See the red line at the top? Make your HR do that. The dotted line is the LTHR for comparison.

Frankly I still started too hard even in that TT (it's hard not to), but there is nothing about accelerating slowly. Get up to speed as fast as you can and stay there. There is a difference between "don't start too hard" and "slowly accelerate." The longer you spend getting up to cruising speed, the longer you are NOT at cruising speed. Accelerate hard but know your limits. If you have never gone 23 mph for 15 minutes, don't get up to 25 mph in the first 4 - you are destined for failure. Know your limits, and then push after the first four minutes.

Remember. Acclerate hard, but don't start too hard.