why do "long runs" kill my legs so much?

Cause you're a pansy.

New shoes with good cushioning, shock absorption and support might help greatly. But really running 5 miles at a time is plenty for exceptional fitness and you're leg soreness will be minimized.

First, let's assume you don't have problems with hydration, alimentation, sleep, and so on.

Second, let's remember that different things work for different people.

[And third, what race distances are you looking at? And what are your race times?]

At ~8:20/mile, assuming that's significantly slower than the pace you run in races, you're really only getting general aerobic conditioning: Your movement pattern at 8:20 is likely very different from your movement at race pace. In that case, you can do alternate aerobic work and probably get a similar benefit. IF you have access to the equipment, you might even want to do something like an "alternate tetrathlon" on your 80min days: 20min for each of four kinds of aerobic exercise in rapid succession (exercise bike, rowing, uphill walking on treadmill, and water running, as one example).

But if that's not realistically available, I can think of two different strategies:

1) Break up your long day into two or even three sessions. I remember well one day when I was coming back from a layoff: 30min in the morning, 40min at midday, 20min in the evening. I got 90min of running at a decent (moderate) pace and felt great, whereas a 90min continuous run would have just been a slog, or even undoable.

2) Do the long run in one session, but insert walking breaks. (I know, I know: "Gallowalking! The horror!") Try running for three minutes, walking for 30 seconds, and repeating. The key here is to take walking breaks right from the beginning of your training run--the idea is to *avoid* fatigue, rather than feeling like you have to "earn" the walk by getting tired first.

Beyond that, consider going a bit faster on your long runs (or your long run/walks). I know it may sound counterintuitive, but I've coached some runners --who really felt terrible on a distance run--to try going faster. As soon as they sped up they felt more comfortable, ran more efficiently, and were less fatigued.

Finally, consider the possibility that you're someone who just doesn't benefit from long training runs (or maybe not even from any high-volume day of running and/or alternate-aerobic work)--even if you're training for a long race. The two activities are *not* identical, but when I couldn't run anymore and moved to 50km walking races, I found that my most helpful training was just to go out for an hour or a bit less, once (or twice or occasionally thrice) a day. Long single training walks made me too tired for the next couple days, and I often gained a pound or two as well; for the last marathon I walked, I don't think I ever went more than five miles in one training session, but in the race I did great.

Be willing to experiment. If you really aren't adapting to 80+min runs, your body is telling you something. Try something else!

Maybe you are running too fast? Try going slower. Like the person said above you need to experiment a bit to find out what works for you. Try alternating your running terrain. One week roads, the next week trails , the following week treadmill. Perhaps take more breaks. Every 5-10 k take a break. Every week build up taking less breaks.

Keep us updated.

There is no reason to conform to the weekly Sunday long run. Try only going long every 10 days or every other week. Also, some lower body strength training may help; half squats, calf raises, lunges, band exercises.

The soles of your shoes are probably soft, which is what makes your legs tired.

The best soles for running shoes have lightweight hard rubber, which doesn't cause jarring in your legs.

Another problem with soft soles is that they absorb the energy, so you're losing energy while not going anywhere.

I do road races, half marathon and below, caring more about 5k and 10k. 5:50ish pace for a 10k right now (estimated). That means my easy pace is over 2 min slower than my 10k pace, which seems easy enough. I've tried running faster but it really just makes me more fatigued. I definitely notice that if I do a lot of miles in a day, but not continuous, like a long warmup and cooldown and a tempo, which can easily get to 10 miles, I definitely feel tired the next day but my legs aren't dead / fatigued in a really unpleasant way. So doing two medium sized doubles would probably feel good compared to a continuous run, but I haven't tried it.

Put simply, that's why they invented doubles. Also, don't believe the nonsense that a long slow run is recovery day.

For track, a monster long run is unnecessary. For thon, build up to it along with other mileage sources.

You need to buy a second pair of running shoes, OP.

One pair for shorter runs

One more cushioned pair for longer runs

And then accept that of course you will be more sore after much longer runs, because you just ran further.