1. Running fitness isn't the same as cycling fitness. I say that as a triathlete. I'm shit at running. Super slow. I hate it. The bike is my best part. No amount of training can turn someone who has a natural facility for one but not the other into someone who excels at both. If you do excel at both, may I recommend a trial with one of the national multisport teams.
2. This sounds a lot like a mismatch between expectations and reality, compounded by reality being altered to meet expectations for a period.
3. The way you say ladies throughout makes me wonder what the gender balance of the club is. Were you going out of your way to be welcoming because the club is heavily skewed towards male and you were trying to address an imbalance? Might that have led you to persevere with them longer than you would have done had they been men? It really jumped out to me that you refer to your existing club members in a way that makes it impossible to determine the gender (other than the chairman's wife, whose identity is given in relation to the chairman, presumably her husband, rather than being "an existing female club member" or even just "an existing member" who also rides in the slow group, and the chairman is her husband so she would have had opportunity to mention it to him).
4. The ride was previously called the Social? Was it called that when they joined? If so, that comes with an expectation it's about pootling along having a chat, rather than trying to achieve a particular speed. It also suggests it's a group that even the racing snakes could join if they didn't want to beast themselves but wanted to focus more on the social aspect of club membership for a change. If the ride is a social ride, it shouldn't be about keeping up. Or, if it requires keeping up, then expectations should be set at the start. "Yes, we have a social ride, but this is a racing club, so even our social rides average around 14-16mph (23-25kph), which is not slow, and you would need to keep up with that. We might wait for you the first time so you don't get lost on your come and try, but if that's too fast for you, may I recommend joining a local Breeze ride? I can give you the details."
5. Of course they objected to the name change. If I joined a club that had a social ride, where the group waited for me, where the focus was apparently on spending time together, and then changed the group to having a target speed, which wasn't what I signed up for and which I knew I wouldn't be able to meet, I'd be upset about not being asked my opinion as well. It sounds to me like they thought they were part of the club but the rest of the club always treated them as the newbies and not proper members.
6. Cycling clubs have a reputation for being aggressive and niche-ridden, and unwelcoming to women. This won't have helped. However, if you have a club website, I think you could probably help yourselves a lot by having a comprehensive FAQ that explains what all the different groups are, what the expectations are for each, and clearly setting out the drop policy. Base this on what the various rides actually do, rather than what they are expected to do, or what they might be prepared to do until they get fed up.
7. I don't think this is your fault, but it's not the women's fault, either. They appear to have developed a misunderstanding of the club based on their initial experience. It is most likely not the club for them. At that age, they are either menopausal or post-menopausal, and that really affects your ability to get fit for speed and hills. Most women don't know how to maintain their fitness through that time of their life, never mind increase it. Even if those run times are accurate, cycling is a whole different kettle of haddock (see 1), and it would take concentrated effort, decent nutrition, focused cross-training, and determination to go from being an easy pootler to keeping pace with a faster group, particularly if group riding skills are also lacking. I feel for them. They need to join a club that is specifically geared towards people like them, and that means finding one that matches what they want to get out of it -- I suspect for the two who are most upset, this was really about safety in numbers, not having to think of their own routes, not having to think about navigation, and having something to get them on their bikes at a specific time rather than doing something else instead.
"Could I join your club?" should always be answered with, "Well, what are your goals in joining a club? We may or may not be the club for you."
Sam