I'm guessing that was the 'Donut Ride'. I've been doing that ride for about 4 years. Once you get to know it, you can try riding tactically to avoid getting dropped.
Here is a power output summary of a typical ride: (I like to work hard on the ride)
1:10 @ 145 w AP, 226 w NP
0:30 @ 287 w AP, 351 w NP (flats and false flats)
0:05 recovery pace
0:25 @ 296 w AP, 358 w NP (rollers and false flats)
0:08 @ 121 w AP, 143 w NP
0:15 Rest Stop
0:15 @ 119 w AP, 155 w NP
0:28 @ 305 w AP, 350 w NP (flats, false flats, rollers)
1:20 @ 114 w AP, 153 w NP
Basically, it is a 3 x 30' ride disguised as a 5 hour ride. Notice that the VI for the hard sections is often >1.2
One of the hardest parts of the ride is the first 'roller' section where you are alternating between L6/L7 and L2/L3 (20-40" on, 80-100" off). This could be done on the trainer.
One of my favourite trainer rides last winter was a 3 x 30' SST session:
1) 30' at 91% FTP steady
2) 30' with over/under averaging 91% (for example, 1-3' at 25 w above 91%, 1-2' at 25 w below 91%
3) 30' @ 80% FTP w/10-15" surges reaching >200% FTP every 3 min (brings the AP to ~90% FTP)
This would also do a good job addressing the demands of a fast group ride.