I did part of the Silk Road last Summer: Iran (entering from Turkey), Turkmenistan and Uzbekistan.
Regarding Turkmenistan: The shortest route from the Uzbek border to the Iranian border (Farap to Sarakhs) is 470 km long, not 550 km. It's tough to cycle the route in five days, but it's possible.
When we entered from Iran, we lost about half a day for the border crossing, most of it on the Turkmen side. It's not that the border was incredibly busy, but they like to keep you waiting. We also spent one morning exploring the ruins of Merv (very much worth it) and we still would have made the crossing within the five days. What stopped us in the end was stupid accident on the last day, which landed me in a hospital in Turkmenabat.
Awesome, that's really good to know! And I'll be sure to check out the ruins! If you have any other recommended sites/towns to see, please let me know!