I've only done as far as a half. I tend to do my longest runs by time. My longest ones last time were just over 2 hours, I ran just under 2:06. Is there a benefit to stretching those long runs to the 2.5 to 3 hour mark? If I'm not doing HM or other quality and just doing them slow, 16 miles will take me about 3 hours.

For me, a runner exactly 18 years older than onemile (meaning an older runner), and one who runs 4 marathons on average per year, doing lots of speedwork is hard. My legs simply cannot take it. They break down. Maybe other men, or women, my age can do it, but I'm built differently, I guess. So because of that, I feel I have to rely on more LRs to give me the endurance necessary for the marathon. That, and strong weekly mileage as well. I still do speedwork once a week, but I don't overcomplicate it. It's either 5-7 HMP mpw or some 800m intervals at 5k pace, alternating, or in phases. I'm not looking for the magic recipe for speedwork at this point in my running life.

I don't think anyone would question your training or racing efforts...well there was one who did. 

For me, a runner exactly 18 years older than onemile (meaning an older runner), and one who runs 4 marathons on average per year, doing lots of speedwork is hard. My legs simply cannot take it. They break down. Maybe other men, or women, my age can do it, but I'm built differently, I guess. So because of that, I feel I have to rely on more LRs to give me the endurance necessary for the marathon. That, and strong weekly mileage as well. I still do speedwork once a week, but I don't overcomplicate it. It's either 5-7 HMP mpw or some 800m intervals at 5k pace, alternating, or in phases. I'm not looking for the magic recipe for speedwork at this point in my running life.

See, I would say running 16 miles on Saturday followed by 20 miles on Sunday would be too much for my legs.  That seems a lot harder to me but I tend to feel more beat up from long runs than speedwork I think.

I've only done as far as a half. I tend to do my longest runs by time. My longest ones last time were just over 2 hours, I ran just under 2:06. Is there a benefit to stretching those long runs to the 2.5 to 3 hour mark? If I'm not doing HM or other quality and just doing them slow, 16 miles will take me about 3 hours.

I'm no expert, but according to the Hansons, 2.5-3 hrs is in fact the most you should ever do for marathon training. The idea being it beats you up to the point where you cannot recover well enough for your quality sessions. So their Advanced plan caps the LR at 16 - again this is for marathons. Not sure what they recommend for HM.

It's not what I'm doing right now though: By following the Hansons, no more speed work . I just enjoy both volume and speed work, so try to keep doing both if possible.