I am looking at something similar for my next 3.5 game. While the edition is different I think some of the ides may also work well for you.
1. Leveling up. 3 months for martial characters and 6 months for casters. The theory being it takes times to learn new combats techniques; it takes time to learn new spells. This will easily add 10 years onto a 3.5 campaign and 15 years onto a 4.0 campaign.
2. Travel. It takes a good deal of time to walk from adventure site to adventure site. Make sure to add in things that slow down travel like rain, broken wagon wheels, paths that are only accessible during certain times of the year.
3. Weather. Rainstorms, Snowstorms, Hail, Tornados, Hurricanes. These have a tendency to stop the adventure for short periods of time. Use them. You can even use them during the wrong time of the year as a catalyst to adventure. Why is a snowstorm occurring in the middle of summer?
4. Seasons. It is winter here in Minnesota. For 4 months we don't do a whole lot. The same can hold true in the game world. Winter is great for stopping the adventure for periods of time. Planting seasons, rainy seasons, etc. also work well.
5. Gathering Information. It can take a lot of time to talk to people or research information. The more that the PCs have to do this the longer it will take them to go from level to level. I always found it strange in 3.5 that gathering information was done by rolling a d20, adding a modifier, and comparing it to an arbitrary DC. And you learn everything in 1d4 hours! It should take much longer in my opinion. For quickly learning information that is a fine mechanic but if the PCs want to do it right it should take a couple of days if not weeks to learn the information.
6. Family. Play it up. Have the PCs meet love interests. Have parents, siblings, and other relatives for the PCs to visit. Maybe there are special days important to the family, tribe, clan, whatever that the PCs are expected to be at.
7. Jobs. Adventure is what they do when they are not being a baker, farmer, judge, etc. Maybe they are just everyday people who get caught up in adventure from time to time.
I hope some of these ideas work for you. They have certainly helped me alleviate the 20 levels in 3 months problem I faced.