John-
I did a single-day 296km ride/race this September (the LOTOJA -- Logan, Utah, to Jackson Hole, Wyoming). I just rode to finish, because it was my first season of road biking. Here's what I did to get ready:
1) I had a similar base level of riding as you have when I started training in earnest -- about 3 months out.
2) I kept up my normal riding routine (10-30 miles/d) during the week w/ commuting and lunch / afterwork rides.
3) I started the 1st weekend training w/ a longer ride, about 50 miles.
4) Then the next weekend, I did a century.
5) I alternated subsequent weekends w/ 50 miles one weekend and a century the next.
6) I made sure my last century was 2 weeks prior to the LOTOJA, and I only commuted to work (at a spinning cadence -- no power sprints) the three days before the ride (no extra riding).
7) During training, don't EVER forget recovery time, because time off the bike is as important as time on. Plan in 1-2 rest days/ week.
On this schedule, I averaged about 150 miles per week. By the time I hit the LOTOJA, I had 5 centuries under my belt, and was feeling strong. I finished the ride (it was on a Saturday) and was feeling good enough to ride to work the next Monday.
[The LOTOJA is a major event in the Intermountain U.S., w/ over 900 participants this year. They have some great long-distance ride training tips on their website:
http://www.epek.com/LOTOJA/]
Cheers,
James
P.S. - Also, don't forget to consider the terrain you'll be riding in. Try and find similar topographic variation wherever you ride to mimic the Swedish terrain.