I've cooked a pot roast many times in a dutch oven but have never attempted one in a slow cooker. I'm a complete novice when it comes them. Many of the recipes I've looked at suggest about 4-5 hours on high or 8-10 on low. I've gotten a late start today and won't have time for the low 10 hour method. Will I end up with a tough roast if I do 3-4 hours high and then a couple on low? I really only have about 6 hours to work with. The cooker have is old school and my understanding is that they don't run as hot as the newer models.