ISDE and Scottish Trials are similar type events in the sense that there is a course established along which there are "trials tests", "skills tests", or "special tests" that are scored/timed. The event itself is not a race. Points are calculated based on how well the rider performs on the special test sections.
A Timed Enduro is an older more traditional Enduro format that was used for decades where a course is laid out and there are "secret" check points along the course. The course is set up with speed changes throughout and average speeds must be maintained throughout the entire course. Those average speeds change throughout the course as well. Sometimes there will be easy sections with slower average speeds, and others more difficult sections with faster average speeds. Riders leave the start line in 30 second or 1 minute intervals in groups of 3, 4, 5 bikes etc. At each check point they determine if the rider is ahead of or behind the "minute" they should be arriving based on the minute they left the start line. Typical scoring is 2 points deducted for every minute ahead of time, and 1 point for every minute behind, thus encouraging the rider not to speed but to try and maintain the most consistent pace for each section between checkpoints. There are rules regarding check point placement etc, which helps the rider determine the most likely placement of the secret check points. An experienced rider in this format can keep time and maintain pace with a trip meter and a watch, however most enthusiasts simply buy jart charts which have everything calculated for them. You can also program times/alarms, and checkpoint locations into an enduro computer.
Modern enduro formats have been following traditional GNCC/Hare and Hound Scramble formats as they are simply easier and I suspect the traditional Enduro format was decreasing in popularity with amateur/enthusiast riders.
The Web Foot used to be a Timed Enduro format, and commonly regarded as one of the most difficult enduro`s in the PNW. The Cowbell was the more friendly event. The Spark Plug also used to be a Timed Enduro format. The first year I rode the Webfoot was 1984. The original course was laid out as a difficult one as it had been dry out for weeks. The 2 days prior to the event and the day of the event it poured, and the course was simply insane. I didn't finish it, along with probably 2/3rd`s of the riders that started it. 1985 was much better. I did finish but I forget how I did. My goal was to simply earn the pin for competing the course. This was long before they shut most of Walker Valley down for several years.
The last Timed Enduro I rode was the Spark Plug Enduro in 2005 I think, riding my 1983 IT250. Short course was somewhere around 50+ miles. I finished in the top 20 of the sportsman class, and rode the last few miles on a flat rear tire. My brother rode his 98 YZ250 in the same event and finished in the top 10 in the sportsman class. The attrition rate in Timed Enduros is actually pretty high.
I quit riding organized events for quite some time as people started riding "poker runs" like they were GNCC events, but wouldn't actually ride or race the actual competition events.
Just moved back to WA from the Midwest summer of 2015, and just picked up a 2013 EC300 R a week and a half ago.