If you are seeking great cardio, look no further than Heathrow Airport! No matter which terminal you are flying in/out/thru, you must walk through long corridors after long corridors. Even after flying through many airports, I still believe no airport requires as much walking as Heathrow does.
As London's main international gateway, London Heathrow (LHR) is most likely everyone's entrance or exit from abroad. The positive aspects of this airport are:
* Cleanliest in every corner: Few airports in my experience are as immaculate
* Top-notch airport lounges: Whether you have airline status or Priority Pass, you will find one of the world's best airport lounges here! Still today, one of my best lounge experiences belong here
* Friendly & efficient airport security: What may be an oxymoron for the American TSA exists in LHR where the security is efficient, provides friendly guidance (and even plastic bags for liquid containers!) and does not waste anyone's time
* Terrific city transfer: Despite its distance from central London, LHR has few terrific city transfer option to fit all budgets
However, while LHR is a decent airport to fly in and out (if you don't get ensnared by the often long customs queue as a non-UK or non-EU citizen), it is a nightmare to transit - even if you are flying on the same airline. I finally had the opportunity (or misfortune) to transfer flights at LHR, and it was an ordeal.
In my case, I flew from Houston to Prague via London with both flights operated by British Airways. Because the Houston-London flight landed in Terminal 5, I must reach T3 to transfer. I already had researched the inter-terminal procedure beforehand but even at the moment, I was shocked at how third-world the whole process was. After we landed at T5, I had to walk through a long corridor to a light rail, walk through another long corridor to the bus stop, take a bus to T3, go through airport security, walk through another corridor to my boarding gate where I board another bus to the plane. If my previous flight was delayed in any manner, there was almost no way I would've made my next flight. As you can see, you should try to avoid a tight Heathrow connection.
LHR is an airport of contradiction. Some of its aspects are incredibly modern and world-class, whereas others may feel third-world. Unfortunately, especially if you are flying to or from overseas, you must go through Heathrow. In that case, schedule plenty of time before you depart, after you arrive, and between your flights because you will need to walk and queue a lot to get anywhere.
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TRANSPORTATION:
LHR is connected by two main rail services: the London Underground (aka the famous Tube) and the Heathrow Express/Connect.
The connecting tube line is the Piccadilly Line, which runs directly through central London locations like Piccadilly Circus and the Covent Garden. The duration to reach from T2 to Kings Cross is about an hour at the cost of £6.9 on peak hours and £4 off-peak. However, Piccadilly Line is one of London's oldest and the train is the smallest I have rode on, so it can be a stressful ride if you are slightly taller and have even just a carry-on.
Heathrow Express (HEX) is a more luxurious and faster option that reaches Paddington Station non-stop in 15 minutes from T2 and 3. However, not only do you need to transfer to the tube if your destination isn't Paddington, the cost can be as high as six times the tube price! But if you order online at least 30 days before, the cost is significantly lower and the 90-day discount even makes it as cheap as the tube.
Heathrow Connect is a slower version of HEX with three stops in between, adding an additional 10-15 minutes.
Which one to take? If you can order in advance, go for the HEX. It may not be the smarter way as the slogan claimed, but it's definitely the much more comfortable way.
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tl;dr version:
1) Insanely huge airport that requires a lot of walking
2) Travel lightly and plan to be there at least 2 hours before
3) Extremely efficient security checkpoints
4) Take the HEX if you can order 30 days in advance read more