Yeah, I agree with Roger.
Generally, you have to have a good starting point. And you need a good exit point, even if it is the end of the trail, you still have to get home. I usually trace my route with a highlighter, but, there is always the side trips and alternate routes along a longer trail. Inbetween starting/stopping, there is too much variability to say I will be at Point A on This Date at That Time. I rely on communications, verbal between other hikers, telephones, and spot checks (whenever possible) for that.
There are a lot of trails out there. Choosing to do a long distance one means some research. YOU will be hiking the trail and it will do you no good to have someone map out a route, FOR you. Generally, trail maps, and topo maps will still be needed because they are needed on the trail, and, these are readily available (well, usually.) You need to start familiarizing yourself with the trail, with or without an itinerary. Indeed, an itinerary may not be possible untill AFTER familiarization. And, hikers do not always maintain 20mi per day. Sometimes you make only 12, depending on conditions. You can never count on conditions.
Current Conditions can change a trail. Wild fires along the PCT. Impassible water in the Grand Canyon. Blowdowns in Vermont. Slides burring/washing away trails in NY. This would be more interesting, but a month or two could well change current conditions and would be costly to maintain in a large database of trails.
I am not sure a service to provide this really has a place in the hiking community. There are simply too many variables, right down to a hiker deciding to take a zero somewhere just because it's pretty.