IMO there are two numbers you need to know. Your lactic threshold and your maximum aerobic function or MAF. (Maffetone term)

Do intervals above LT and train below MAF for recovery.

You say you feel you can't keep the pace at above 160. Your 85% MHR from your numbers is 164 so that makes sense. You're at your limit, or almost.

According to Maffetone, your MAF is 180-age which puts you at 139, but it seems like you have a higher HR than that predicted by 220-age. I'd probably add 5 BPM to that, no scientific reason, so 144 or almost 70%. I'd stay below this for recovery. Kinda falls into place doesn't it?

Anyway, do your intervals. I wouldn't worry about a set amount of reps or time, just decide one day that you're gonna go do hard intervals and keep them under an hour total WORK time. In fact, for the first set of intervals, I recommend keeping the work under 15 minutes. Do them above LT, and use a ration of 2:1 work to rest. Start with one interval session every 10 days for a month, then once a week for a few weeks.