That seems to be an inquiry about how Anna Feodorovna got from being Juliane of Saxe-Coburg-Saalfeld in Bavaria to the Russian Tsar in 1795?
In that case, she travelled overland, by coach for quite a few days.
A description in novel format of that specific journey naimng a few stations is found in Therese Bichsel: "Grossfürstin Anna: Flucht vom Zarenhof in die Elfenau", Schwabe, 2013.
They started on 12 Aug 1795 and arrived two months later.
If the concrete example above shouldn't suffice:
Going by boat from way inland Coburg is not very practical. Nearly all major navigable river systems head North-West, emptying into the North Sea. Going around Denmark would certainly add a huge delay. Only Oder and Vistula are on the way to St Petersburg and flow into the Baltic. That would have been a possibility, but one with questionable advantages. Both rivers add a natural flow again away from the destination by a large degree. You'd also have needed at least change vessels once: embarking on a river boat, then change to sea-worthy.
The theoretical maximum travel speed at the time on land would be achieved by stage coach. Stagecoach travel was designed with speed in mind. Stages would average 60 to 70 miles in one day.
For ideal conditions in mostly German lands that would have meant up to 100km a day. (Klaus Beyrer: "Des Reisebeschreibens „Kutsche“. Aufklärerisches Bewußtsein im Postreiseverkehr des 18. Jahrhunderts", in: Wolfgang Griep, Hans-Wolf Jäger (Ed.): "Reisen im 18. Jahrhundert" Winter: Heidelberg, 1986.)
This would make the optimum distance of between 1834 to 2400 km
click on maps for larger view
a roughly 20 day trip. But that optimum could not be achieved in comfort, with entourage (only strictly necessary baggage needed?), excludes sight seeing, diversion, visits, detours, and discounts for the minimum of 4 border crossings
and actual road conditions. These would deteriorate somewhat the farther East the trip gets and be reduced in options. Meaning fairly early on the trip, shortly after crossing the Elbe river. Just adding stages to be on major towns, which would be probably a requirement for higher-up in the social strata passengers, within the modern road system extends easily the journey to much more than 2600 km.
For the first two thirds of the trip, clinging to postal roads would map a trip along these lines:
Setting the first stages most probably at a route from Coburg – Judenbach – Grefenthal – Saalfeld – Neustadt – Gera – Zeitz – Pegau – Leipzig
But as that is a journey through lands of closely connected ancestry, visits to grandmas and uncles would be likley?
Then on to something like this
arriving in the Russian Empire: