Normally the verb comes first only when a question is being asked. Why is the verb in the first position in this phrase, even though it's not a question?
Laufen ist eine gute Aktivität.
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This is not a verb. It is a noun: Das Laufen. (English: the running.)
But it's a special kind of noun. It's a nominalized verb. (German: Substantiviertes Verb). But this term is misleading. A better term would be verbal noun (German: Verbalsubstantiv), because this term would correctly name the noun as "noun", but still the term "nominalized verb" is much more common.
But still, there are sentences in German, that really start with a verb in position 1:
Closed questions (questions to be answered with yes or no)
Laufen Herbert und Susanne morgen auch beim Marathon mit?
Are Herbert and Susanne also running in the marathon tomorrow?
Laufen Sie nach Norden!
Run to the north!
Conditional sentences without wenn:
Esse ich viel, dann habe ich großen Durst.
(Version with wenn: »Wenn ich viel esse, dann habe ich großen Durst.)
Irreal wishes without wenn:
Würde es doch endlich aufhören zu regnen!
(Version with wenn: »Wenn es doch endlich aufhören würde zu regnen!)
very important participle or infinitive:
normal structure, the important part is at the very end of the sentence:
Er hat schon lange nicht mehr geschlafen.
Susanne und Martin werden im Herbst heiraten.
very important Verb on position 1:
Geschlafen hat er schon lange nicht mehr.
Heiraten werden Susanne und Martin im Herbst.
Laufen drei Ärzte die Straße entlang. Sagt der erste: "...
Your question is caused by confusion about the words "the verb" and "first" in the syntax rules. You were drawing correct conclusions from incorrect descriptions. Such sloppy description of the German syntax rules, unfortunately, have become the norm. Even the answers given above mostly seem to be unaware of this point.
The syntax rule is actually not about "first / second", in the sense of relative ordering. The rules of German clause structure are about two fixed positions which introduce the German clause; common names are "Vorfeld" (prefield) and "linke Satzklammer" (left bracket). The prefield can be a unit as big as you want, the left bracket is a single verb. A "verb-first" clause is one, in which the prefield is empty, so the word in the left bracket is the first word you see. A verb-second clause is one which has material in the prefield, so "the verb" (but see under 2) is the second unit – but whether "first" or "second", it is always in the same position in absolute terms. It is preposed (instead of being in the clause-final slot).
It is sloppy and misleading to say "the verb" comes first/second, because this rule is not about "verbs", it is only about the finite verb form. There is only one finite verb per clause, although there can be many additional verbs in infinitival forms. It is misleading to speak about "the verb" when there can be several verbs, isn't it. What the rule means (but doesn't express properly) is exclusively "the finite verb", this one deserves the article "the" because this is unique. So the finite verb of a main clause goes into the left bracket; and in some clause types the prefield must also be filled, to make it a "verb second clause". But it is not really "verb-second", it is actually "finite-second".
Infinitives can always go into the prefield in German, but this does not bear on the verb-second rule if understood properly. It is not necessary to consider "Laufen" to be a noun in order to get the syntax right in your example. Even bigger infinitival phrases can be in the prefield:
(the second example shows an object in accusative case, so it must be a verb what you see in the prefield...)