1.

You observe that someone is approaching from the opposite direction.

towards

You observe that someone is coming TOWARDS you.

2.

You both halt and exchange apologetic smiles.

each other

You both halt and smile apologetically at EACH OTHER.

3.

That we are aware of its importance can be gauged from the significance parents attach to a baby's first steps.

know how

That we KNOW HOW important it is can be gauged from the significance parents attach to a baby's first steps.

The significance parents attach to a baby's first steps shows that we KNOW HOW important it is.

4.

With the exception of talking, no other developmental landmark is so eagerly awaited.

only

Talking is the ONLY other developmental landmark that is as eagerly awaited.

5.

They filmed four hours' activity on one such crossing.

kind

They filmed four hours' activity on one crossing of this KIND.

6.

Women turn their backs on them, regardless of their own age and the age of the other person.

however

Women turn their backs on them, HOWEVER old they are and the other person is.

7.

In the next few years we must expect to hear a lot more about walking.

likely

In the next few years we are LIKELY to hear a lot more about walking.

8.

We must expect to hear a lot more about walking from psychologists and sociologists.

tell

We must expect psychologists and sociologists to TELL us a lot more about walking.

We must expect that psychologists and sociologists will TELL us a lot more about walking.

9.

It will sound so complicated a skill that many of us will hesitate to set foot outside our front doors.

such

It will sound SUCH a complicated skill that many of us will hesitate to set foot outside our front doors.

10.

This won't deter the five million Britons who treat walking as a recreation.

put  /  think

This won't PUT off the five million Britons who THINK of walking as a recreation.