When combined with a couple of shorter runs (6-8 miles) per week?
I've been building my base for 4 weeks, and I have been seeing improvements, but no maginficant jump in endurance like I would have hoped. My longest run up until now has been around 60-70 minutes.
If I build up to 2 hours as my long run, will I see a significant improvement in endurance?
You havent seem the jump because its been 4 weeks (i.e. not really that long). Build to 2 hours, and because it will take you a few weeks to get there, if only because of that, you should see gains.
When building base worry about total mileage first, long run second. You will see the most significant improvements with week after week of high mileage, whether you have a long run or not.
This is not to discount the value of long runs. They are useful. But volume is more important
make your long run up too 25% of your weekly mileage
bumppp
volum is obviously important, but i find the long run to be the most important run of the week while base training...