I’m guessing it is putting you on a Low Volume plan based on how much training you have been doing recently. So one option is to game it and put in a higher training volume so it gives you a recommendation of MV or even HV, risk of doing that is that you can’t handle that much structured work. Other option, which I think is better, is to use that 3-6 hours as the foundation of your training, but then increase your volume by supplementing with zone ~2 rides, including a long weekly ride which you can steadily increase the length of.
Also think you’d be surprised at how well 3-6 hours of very structured training can prepare you for a 140km event at least. Though agree that 350km will need a bit more volume!