I'm a firm believer in not mixing speed and distance in my training. Both put stress on ones body, and I think it is wise to subject our bodies to stress from only one source at a time. So, I usually do LSD for my distance running, or do fartleks and intervals for my speed training, but not both in the same runs.
However, as you're aware if you've been following my blog, I've been increasing the distance of my Tuesday rest run and also letting my body go as fast as it wants to go. This means that I'm mixing distance and speed together. I'm doing this because I've been making small increases in my distance: approximately 1 mile per week added to my Tuesday rest run. One mile increase in 30 miles run per week is ~3% increase per week. I figure that is a small increase and that my body can handle the stress from that as well as the stress from running faster. Also, I'm letting my body decide about running faster, so when it wants to run faster, that is ok because it is doing well in handling the stress from the small increase in distance and the stress from running faster. The way a person gets into trouble by mixing speed and distance is when the person's brain, not the person's body, decides to run faster even though the body may not be doing well in handling the stress from the faster runs.
Tomorrow will be my last Tuesday to add distance. I'll add 10 minutes, and that will give me another hour rest run. Then, I'll start a new rest run on Thursday, starting with 10 minutes, and I will add 10 minutes per week. So, by mid December my Thursday rest run will be up to an hour, and I'll cap it at that amount of time. That will give me approximately 38 miles per week. I won't start increasing my long run from 13 to 14 and then to 15 miles until February.