I’ve had the same thing in the past. Would feel light headed for hours after and intense workout. For me the solution was in nutrition on the bike. I’ve started taking at least 60 grams of carbs and hour during the ride and that solved it for me. Usually use drink mix (2:1 maltodextrine to fructose) with a pinch of sea salt. But a mix of drink and solid food (I prefer bananas) works just as well.
About the plans, yes. They should ease you into it and certainly not start with heavy above ftp sessions. Not sure what went wrong there but I suggest you contact support and get some help from them.