I've broken them up into two rides in the same day. Over two days you still get in the distance but the benefit of a long ride probably isn't as great. If that's the best you can do it has to work. We all have lives to live and to work around. I've been on the road before 5 am for all my long rides this year to minimize time away from the family.

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1 long ride != 2 medium rides

The biggest benefit IMO to a long ride is getting your body used to moving for that long and getting used to the fatigued feeling while still holding pace and moving forward. Can also be a good test of your long nutrition.

A long ride will inevitably provide a better training stimulus than say two shorter rides at the same intensity if we're talking about long distance racing.

But breaking up say, 100 miles into two 50 mile days and upping the intensity will also provide a great training stimulus that *could* benefit you more than a long ride, unless your body is in absolute need of long slow base miles for training adaptation.

If you have family/work commitments and you're not getting paid to race I'd go for option B. Split the rides up, maybe add some intensity to part of the ride, recover well.

2.5 hours on two days is not the same as 5 hours straight on one day because of the differences in cumulative fatigue during the rides. But, you always have to work within the time constraints you have and "shorter" rides can be killer and, frankly, a relative lack of hard intense short rides are probably the biggest weakness in your typical tri bike training plan.

So, take advantage of this opportunity that has been given you, up the intensity and enjoy your shorter rides!

Just breaking up, say, a 100 mile ride into 2 50 mile rides of the same pace, may be the wrong way to approach this. You may get more out of this by doing something different on your two rides. Or doing something like one 50 miler than doing a shorter interval workout on the other day. Don't just blindly divide by two. Have a plan on what you want to accomplish.

As others have said, breaking it up isn't really going to replicate a long ride, but does probably mean you can do more intensity (subject to overall fatigue levels). In my experience, the main benefits from long rides are psychological and logistical more than physical. I.e. it is useful to get used to mentally being in the saddle that long and coping with the fatigue/boredom, and it is useful to be able to practice your nutrition and pacing strategy, but I don't think it necessarily adds any more to your fitness than 2 shorter rides adding up to the same distance.

First IM I did I was terrified by the distances and did a lot of long (>100 miles) rides and runs (18-20 miles). Second time around I only did a couple of centuries and 2-3 runs over 15 miles, but was more focused on quality and intensity. Knocked an hour off my time (12:3x to 11:3x) on similar overall training volumes, similar body weight, etc. Since then I've done 100 mile bike races with a long ride of 50 miles in training, and not had any problems at all. In fact I did a 100 mile ride on Saturday (not a race but an organised group ride with some roadies who like to keep things pretty competitive and treat it a bit like a TdF stage where you know you're going to come into the last few miles all together but there's still a lot of fun to be had attacking along the way) when my previous longest ride this year was 48 miles. Rode hard and still felt pretty good at the end (and the last 5 miles was pretty much a balls to the wall effort with half the group dropped by the time we got home). I'd feel happy that if I needed to I could train for an IM with my long ride most weeks being 3 hours or less, just as long as I was getting the total volume in through frequency instead.