One of the problems with the Heathrow set up - and for many other airports - is that pedestrian data is often wrong or misleading. So walking from the Hilton T5 to Terminal 5 will show a silly route, to give one example of many, but if the weather is nice then there is a perfectly reasonable and shorter walk. Many airports simply don't put in many pavements around them, JFK is one glaring example. In this case it really is straightforward, a straight line with a footpath set well over from the road traffic, and it's a walk I have done a few times.

I doubt it will take as long as 35 minutes. One of the other factors about Google Maps is that it doesn't greatly distinguish between different sorts of pedestrians, and just puts down a slow average speed. The reality is that once you get to over 1 km the sort of people willing to walk that distance will probably be fairly fit and thus will walk it in a lot less time. I'm an OAP and it takes me less than that, and the OP here hasn't got their A bit yet.