There are several problems here.

1. We don't actually know how big Tamriel is supposed to be. TES1:Arena gave a rough estimate of 3-4000 km East-West and 2-3000 km North-South. This is vague enough to be of rather limited helpfulness. And it's not entirely clear how much later games felt bound by this.
2. We don't really have an authoritative map of Tamriel. We have various iterations in the games, we have various maps published alongside. While there is a shape of Tamriel that is fairly consistent in maps from TES4:Oblivion onwards, they differ somewhat from the one used in TES1:Arena, so it's not entirely sure how those measurements are applicable. They're also more for establishing a general picture of scale, not very good for measuring smaller distances.
3. Ingame maps can differ substantially from the general world map, both in size and shape of regions, where individual map points are situated in a region, and the general "feel" of distances.
4. And something that has nothing to do with maps: speeds of sailing ships are highly variable, depending (unsurprisingly) on wind conditions. With favourable winds and no obstacles, 6 knots (nautical miles / hour) are entirely possible; in bad conditions that can slow down to below 2 or zero.

That said, if we assume
- that TES1:Arena measurement Tamriel to be still relevant, and
- that the latest maps like the one from the Elder Scroll Anthology (UESP Link) are more or less accurate,

then we get a rough distance between Northpoint and Daggerfall of about 1000 km (yes, a thousand), and for sailing between Daggerfall and Sentinel another 700-750 km. (Very rough estimates with the measuring tool and grid in IrfanView, so don't take them as gospel, but should give a vague idea about the distances and times involved.)

Which means:
- If we assume good conditions with the prevailing wind and sailing clear of the coast, the voyage from Northpoint to Daggerfall could be made in 4 days if sailing full speed day and night, but probably slower. That could slow down to 10-14 days if conditions are worse or they hug the coast.
- From Daggerfall to Sentinel, theoretically maybe 3 days in very favourable conditions. But as this is a voyage with changing directions and the entrance into the narrowish Iliac Bay which probably doesn't have guaranteed wind constantly from the most ideal of directions, it's highly unlikely, so 4-5 seems more realistic under decent conditions, still assuming sailing throughout day and night.

Well, other members of the Thieves Guild also make it clear that Vipir is not an "unreliable" narrator, he's actually very reliable in that you shouldn't believe his tall stories.
If we'd believe him and say that Windhelm - Riften is about 50 km (a bit more than a marathon run but with mountains and detours in between), then Tamriel would be about 300 - 350 km North to South. That's ... awfully small.

The continental US are about 4500 km by 2500 km - i.e. roughly the size of the whole of Tamriel, just for comparison. Europe from Lisbon to Kharkiw is a similar distance, though it's harder to compare due to not being such a blob.

Even if we divide Arena's scale in half, the distance between Riften and Windhelm is too far to make it running in a few hours. I think we can safely dismiss Vipir's claim as fanciful exaggeration.