Should I try to stay in my low threshold zone the whole time? That seems to be in the 140 to 145 range for me.

Give it a try. There's a good chance it will become very difficult and slow for the last few miles.

That was weird! My legs just felt like they got overloaded with lactic acid or something. Is that typical? I ended up doing alternate sprints and walks to keep within the 140 to 150 zone. That is probably cheating but about all I could do to keep going. On the good side I guess is that I don't feel fatigued or any pains at all. It gets me concerned about my marathon coming up in a few weeks though. Should I do a few more of these types of long runs? Do you taper the same way doing these low threshold long runs or do you keep doing them right up to the marathon?

That's about what I expected. Don't worry about it at all or read into it. Yes, you should try several runs like that - they do get better. It's all part of developing aerobic fitness. I actually don't taper at all, but you don't want to take my advice on that aspect. In many cases, I've run a 20 miler the day before or the day after a marathon or 50k. It's how I prepare for 50-100 mile races. It takes me about 5 or 6 long runs into a training season until I can hold a solid 20 miles at a nearly steady heart rate (there's always going to be some drift). As a side note, if you're not hydrating well, then you will sink really hard with a few miles to go. If you weigh yourself before and after your run, you'll know. (About 3% weight loss will cause a significant raise in heart rate.)