Starting Point: Weesen Bahnhof
Trail Type: Tough tour for experienced bikers. (May to November)
Physical Fitness: 4 (43.4km, 1290m ascent, 1560m highest point)
Technical Difficulty: 5 (very technical singletrail with some highly suicidal sections)
Map: Ordnence Survey Map Walensee
I found this route on GPS-tracks.com and navigated the route with my Garmin Forerunner 305. The GPS download and much more detail on the route can be found here. You can easily do the route without the GPS though by following the directions:

Weesen Bahnhof - Mühlehorn - Murg - Walenstadt - Walenstadterberg - Schwaldis - Sälserhütte - Hag - Laubegg - Betlis - Weesen

This is a tough tour with 6 distinct sections which are all interesting in their own way. I would only do this tour in good weather with the ground as dry as possible. Although I did the tour alone, I would recommend to only go in a group - there are some seriously difficult sections and it would be good to know that someone was there to "pick up the pieces".

1. Start from Weesen Bahnhof (plenty of parking 50rp/Hour). Cycle on the veloweg along the side of the lake, next to the autobahn to Walenstadt. This is about 20km of flat, mostly tarmac riding that serves as a great warmup and allows you to take in some nice scenery and gaze up to the Laubegg on the far side of the lake where you will be later.
2. The climb! It's tarmac almost the whole way from Walenstadt to Walenstadterberg but it's steep and relentless and feels like a lot more than 1000m. Great scenery though with amazing views down the valley to Chur and beyond.
3. Finally the road ends and you continue on a trail towards Alp Schwaldis. There's a section with a bike verbot but according to the GPS tracks site, bikes are tolerated before 1700 Mon-Sat. There are a couple of shorter climbs that will test your legs again but it's all easily ridable.
4. There's a turn off to Gocht (which you don't take) and after this, it starts to get very tricky indeed. If you look at the trail alone, then most of it would be ridable. The problem is if you put a wheel wrong then you will most likely fall to your death. You will need to carry the bike for quite a lot of this section, riding what you dare in between! In mid-June, I needed to climb over one quite substantial snow drift in a gully which was very very slippy indeed. I actually had to excavate some foot holds out with my hands. Even carrying/wheeling the bike was precarious at times. Good shoes are essential on this ride. This section has some amazing views of the lake.
5. Finally, at about 1100m altitude, you come (walk) down through an abandoned hamlet (there was a tap here where I filled my camelback) and enter the forest where the track becomes more ridable although still highly technical. It's a fantastic singletrack with many tight switchbacks, rocks, roots, drop offs - everything you could want. I had the track to myself on a Friday morning but I would imagine that it would be a nightmare on a sunny weekend. It's very steep and technical and walkers would really detract from the enjoyment. The singletrack goes all the was down to Betlis (there were walkers for the last couple of km but the track is wider at this stage.
6. After Betlis you follow the tarmac path by the lake back to Weesen. There are loads of opportunities to swim in the lake and a few restaurants as well.

I completed the tour in 4'15" including a couple of short stops to eat and enjoy the view as well as chatting with a friendly walker. If you were riding in a group and enjoying the view then I would allow at least 5 hours.

In summary this a must-do tour for experienced MTBers. It's a very enjoyable, picturesque and technical ride and you need to be prepared to carry the bike for some longish sections.

Talking to some other veterans of this tour, it seems as though my enthusiasm for shouldering the bike along cliff edges is not universally shared by all and therefore I'm downgrading this from a must-do tour to a "rather good but quite difficult and maybe a bit frustrating for some" tour.