Short answer: it's 8 minutes walking, or, 1 minute on the tube but then this also includes not much less than 8 minutes walking between underground platforms, including navigating the warren that is King's Cross underground station.
Arrive at the station at least 5 minutes early, and based on Gagravarr's comment, if you're arriving on Eurostar allow another 10 minutes at least whichever option you choose just for getting out of the arrivals area, if you're unlucky! 25 minutes is actually a bit tight, especially if you're arrival is delayed.
It's probably easier to just walk between the two - less stressful, cheaper and barely any slower. If you've got time to dawdle, you'll pass the British Library which is one of London's (many) very good free attractions.
I'm not sure which journey planning site you used to get that from, but you can get lots of detail from the London-specific ones. There are two I'd recommend, both use slightly different approaches:
TFL (Transport For London) is the official one. It estimates 8 minutes by walking, or 5 minutes by public transport for that journey, based on:
- 4 minutes to get from St Pancras to the King's Cross Underground
- 1 minute on the tube (Victoria Line)
Citymapper is a very popular alternative that also has a really good app and covers several other major cities in addition to London. It also estimates 8 minutes walking (and shows a handy map of different routes plus other route options). It estimates around 3 minutes by tube based on:
- Up to 2 minutes wait for a Victoria line train
- 1 minute on the tube
- 2 minutes walk from Euston underground station to the national rail station
So, put those together and if you've got a train to catch, you should allow at least 10 minutes (plus any extra train arrivals time) based on:
- ~4 minutes to get from St Pancras train to King's Cross victoria line underground platform. King's Cross underground is a bit of a warren so I might say allow 5 minutes for a first-timer in case you go the wrong way
- Up to 2 minutes waiting for the Victoria line (which generally has a reputation for being fast and reliable)
- 1 minute on the Victoria line
- 2 minutes to get from Euston underground station to the platform (Euston station is a dismal place, but it's underground is less of a maze than King's Cross)
Then there's the matter of paying to get onto the tube, which will add some time if you don't already have an oyster card, travel card or contactless credit/debit card you can use in the UK.
To be honest, I'd just walk it. Very easy walk with a nice view of the St Pancras exterior.
I just noticed your comment about taking your time: you'll also be walking past the British Library which has many interesting exhibitions (usually free) and the "Treasures gallery" displaying famous and historically interesting books and texts ranging from Leonardo da Vinci's notebook and the Magna Carta to original handwritten Beatles lyrics. There's also a rather nice informal Japanese restaurant between the two stations on Chalton St.