It is hard to say exactly what could be the reason for your difficulty being able to handle the kind of mileage you used to be able to handle. I will try to hit on all the areas that I can, using the information you have provided. Perhaps one of them is the reason. You will have to try each one of them and be patient, progress can be slow.
First off 6 mmol/l in an all out run test is pretty low. This is about what a marathon runner might see when he or she is ready to race. It should be higher if you are trying to increase your aerobic capacity. How far did you run in your all out test? Olbrecht recommends 600m. You can do a 400m test but your lactate levels may be about 1-2 mmol/l lower. Don't run longer than 600m though. If you run for too long your aerobic system will catch up with your energy needs and start to bring your lactate levels down.
How many blood samples did you take after your all out test? You should draw blood within a minute after you finish the run and then continue to take blood at 2 minute intervals until you see your lactate value come down. If you only take one or two you may not have gotten your highest lactate value. It takes a little bit of time for arterial blood lactate levels to catch up to what is actually going on in the muscles.
If you take a long time off from training it can take a while for you to rebuild the mitochondria that are responsible for energy production and endurance. It does not take as long for someone who has recently been fit to gain fitness back though. Your capillary system that you developed from high mileage should mostly be intact so you shouldn't have to develop that.
I would look at your strength. Have your muscles atrophied at all from inactivity. It may take a while for you to gain the necessary strength necessary to run high mileage again. I would look into doing some specific hill work to build your running muscles.
I like to prescribe short hill sprints that take about 15-20 seconds to perform. This is basically anaerobic capacity work but you don't get the pounding that running on the flats gives you. It works your muscles a little bit more also. You need to ease into this though, because you don't want to overstress your Achilles tendon and calf muscles. Start running them easy at first and then increase the intensity a little bit every time you do them. Take about a 2-3 minute walk recovery after each one. Do them once to twice a week depending on what other workouts you are doing.
You could also try some strength exercises using weights. You may be week in some area and need to work on it. Jump rope is a good exercise. So are calf raises and squats. Some hamstring work on one of those big exercise balls might also help. Hamstring curls don’t do much for runners.
As for translating Olbrechts book into running workouts. Basically you need to take the swimming distance and multiply it by 4. So if Olbrecht talks about doing repeat 100's in the pool, you could do the same workout on land but you would do 400's instead (4 x 100 = 400). The same goes for all the other distances. This is also how you can compare race distances. A 200m race in the pool is the equivalent of an 800m race on the track. The time necessary to complete the two distances in a race is about the same.
As for recoveries, you would basically multiply Olbrechts 15 second recoveries by 4 also. So instead of repeat 100's in the pool with a 15 second recovery you would perform repeat 400's with a 60 second recovery. You can take more rest if you need it though.
Olbrecht recommends that you only do a minimum amount of aerobic power work and anaerobic capacity work during workouts in the base phase. He talks about limiting it to 600m in the pool. Again, you just multiply this number by 4 and you will get 2400m, the amount you should do running on land. He has suggested that you can do a bit more than this though, about 2 miles worth, but probably not more than that, and that is only for aerobic power workouts not and anaerobic capacity work. You probably want to limit that to a mile on land. You can only sprint near to all out too many times. If you do more than that in a workout he suggests breaking the workout up by doing about 4 miles of easy running in the middle of the workout.
Have you thought about your diet? Maybe you need more iron (take vitamin C with it).
Maybe you need more protein about 1g per pound of body weight.
Maybe you need more carbohydrates. It is very difficult to increase your mileage if you are already glycogen depleted when you start. The more mileage you run the worse it gets. I kind of think this is probably the one area you would benefit most from if you aren’t already eating enough. Eat high glycemic carbs within the first 30 minutes to an hour after your run, and then try to consume low glycemic carbs the rest of the time. Drink plenty of water with it too. You store a gram of water for every gram of carbohydrate you store in your muscles. A little bit of weight gain would not be a bad idea if this is the reason for it.
From what I hear you saying I would probably work on your anaerobic capacity. Do as much easy running as you can the rest of the time and take days off if you need to. Don’t do any long runs over 90 minutes until you feel you are ready to.
I hope this information helps you.